To reproduce: please refer to the attached gif file.

The error is not reproduced each time with the Demo application. Sometimes an unhandled exception dialog appears and sometimes the application becomes unresponsive and you can not close the application with upper right X, the options to the right does not respond, you can not change demo using the list on the left. Selecting "Settings" in Property Grid section and then clicking on a boolean property (left column), scrolling using the mouse wheel, then selecting another property (with PropertyGridDropDownListEditor) or clicking around in the application (even outside of the property grid form).

Workaround:   this.radPropertyGrid1.EditorRequired += radPropertyGrid1_EditorRequired;

private void radPropertyGrid1_EditorRequired(object sender, PropertyGridEditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(PropertyGridDropDownListEditor))
    {
        e.Editor = new CustomPropertyGridDropDownListEditor();
    }
}

public class CustomPropertyGridDropDownListEditor : PropertyGridDropDownListEditor
{
    public override object Value
    {
        get
        {
            PropertyGridItemElement element = this.OwnerElement as PropertyGridItemElement;
            PropertyGridItem item = element.Data as PropertyGridItem;
            if (item == null)
            {
                return null;
            }
            return base.Value;
        }
        set
        {
            base.Value = value;
        }
    }
}

To reproduce: use the following code snippet and refer to the attached gif file:

 public Form1()
 {
     InitializeComponent(); 
     this.radPropertyGrid1.SelectedObject = this;
 }

 private void radButton1_Click(object sender, EventArgs e)
 {
     ThemeResolutionService.ApplicationThemeName = "VisualStudio2012Dark";
 }

 private void radButton2_Click(object sender, EventArgs e)
 {
     ThemeResolutionService.ApplicationThemeName = "ControlDefault";
 }


Workaround:

private void radPropertyGrid1_EditorInitialized(object sender, PropertyGridItemEditorInitializedEventArgs e)
{
    PropertyGridTextBoxEditor editor = e.Editor as PropertyGridTextBoxEditor;
    if (editor != null)
    {
        BaseTextBoxEditorElement el = editor.EditorElement as BaseTextBoxEditorElement;
        if (el != null)
        {
            el.BackColor = Color.Black;
        }
    }
}

Workaround:
 Private Sub RadPropertyGrid1_EditorRequired(sender As Object, e As PropertyGridEditorRequiredEventArgs) Handles RadPropertyGrid1.EditorRequired
     Dim te As PropertyGridTableElement = TryCast(sender, PropertyGridTableElement)
     If e.EditorType = GetType(PropertyGridSpinEditor) Then
         Dim editor As New CustomPropertyGridSpinEditor
         If editor IsNot Nothing AndAlso te IsNot Nothing Then
             Dim type As Type = RadPropertyGrid1.SelectedObject.[GetType]().GetProperty(e.Item.Name).PropertyType
             If type = GetType(System.Double) Then
                 DirectCast(editor.EditorElement, BaseSpinEditorElement).DecimalPlaces = 4
                 e.Editor = editor
             End If
         End If
     End If
 End Sub

 Public Class CustomPropertyGridSpinEditor
     Inherits PropertyGridSpinEditor
     Public Overrides Sub Initialize(owner As Object, value As Object)
         Dim decimalPlaces As Integer = Me.DecimalPlaces
         MyBase.Initialize(owner, value)

         Dim element As PropertyGridItemElement = TryCast(owner, PropertyGridItemElement)
         Dim item As PropertyGridItem = TryCast(element.Data, PropertyGridItem)
         Dim editedType As Type = item.PropertyType

        If ((editedType = GetType(Decimal) OrElse editedType = GetType(Double) OrElse editedType = GetType(Single)) AndAlso decimalPlaces <> 0) Then
         DirectCast(Me.EditorElement, BaseSpinEditorElement).DecimalPlaces = decimalPlaces
         Me.Value = value
       End If
     End Sub
 End Class

To reproduce: run the attached sample project and follow the steps from the gif file.

Workaround: cancel the edit operation when handling the MouseDown event of RadPropertyGrid:

        private void radPropertyGrid1_MouseDown(object sender, MouseEventArgs e)
        {
            var elementUnderMouse = this.radPropertyGrid1.ElementTree.GetElementAtPoint(e.Location).FindAncestor<RadScrollBarElement>();
            if (elementUnderMouse!=null)
            {
                this.radPropertyGrid1.CancelEdit();
            } 
        }

Workaround use the attached custom theme: http://docs.telerik.com/devtools/winforms/themes/using-custom-themes

To reproduce: use the following code snippet and follow the steps from the gif file:

            PropertyStoreItem item1 = new PropertyStoreItem(typeof(string), "Test1", "Test1");
            PropertyStoreItem item2 = new PropertyStoreItem(typeof(string), "Test2", "Test2");
          
            RadPropertyStore store = new RadPropertyStore();
            store.Add(item1);
            store.Add(item2); 
            this.radPropertyGrid1.SelectedObject = store;

            this.radPropertyGrid1.ToolbarVisible = true;

Workaround: close the editor programmatically when the search box gets focus.

this.radPropertyGrid1.PropertyGridElement.ToolbarElement.SearchTextBoxElement.TextBoxItem.GotFocus += TextBoxItem_GotFocus;

        private void TextBoxItem_GotFocus(object sender, EventArgs e)
        {
            this.radPropertyGrid1.EndEdit();
        }

Steps to reproduce:
1. Add a RadPropertyGrid to a form
2. Set the SelectedObject property to an object with an enum property
3. Open the enum property for edit.
4. Click on another control that can take focus e.g. the form control box buttons
You will see that the editor remains open.

To reproduce:

Public Class RadForm1
    Sub New()        
        InitializeComponent()
        RadPropertyGrid1.SelectedObject = New MyProperties
    End Sub

    Public Class MyProperties

        Private _height As Integer = 70
        <Browsable(True)> <Category("Rows")> <DisplayName("Height")> _
        <Description("Sets the height of the row. Range 70 to 200.")> _
        <RadRange(70, 200)> _
        Public Property Height() As Integer
            Get
                Return _height
            End Get
            Set(ByVal Value As Integer)
                _height = Value
            End Set
        End Property
    End Class
End Class

When you activate the editor you will notice that you are allowed to enter values outside the specified range 7-200.

Workaround:

AddHandler Me.RadPropertyGrid1.EditorInitialized, AddressOf PropertyGridEditorInitialized

    Private Sub PropertyGridEditorInitialized(sender As Object, e As PropertyGridItemEditorInitializedEventArgs)
        Dim spinEditor As PropertyGridSpinEditor = TryCast(e.Editor, PropertyGridSpinEditor)
        If spinEditor IsNot Nothing Then
            spinEditor.MinValue = 70
            spinEditor.MaxValue = 200
        End If

    End Sub

Add a RadPropertyGrid and change its Dock property to Fill. Use the following code:

public partial class Form1 : Form
{
    RadPropertyStore _store;

    public Form1()
    {
        InitializeComponent();
        this.radPropertyGrid1.CreateItemElement += new CreatePropertyGridItemElementEventHandler(this.onCreateItemElement);
        _store = new RadPropertyStore();

        PropertyStoreItem barItem = new PropertyStoreItem(typeof(Int32), "TrackBar", 25);
        _store.Add(barItem);

        PropertyStoreItem sec = new PropertyStoreItem(typeof(bool), "Checkr", true);
        _store.Add(sec);

        this.radPropertyGrid1.SelectedObject = _store;
    }

    private void onCreateItemElement(object sender, CreatePropertyGridItemElementEventArgs e)
    {
        PropertyGridItem item = e.Item as PropertyGridItem;

        if (item != null)
        {
            if (item.Name == "TrackBar")
            {
                e.ItemElementType = typeof(TrackBarPropertyGridItem);
            }
        }
    }
}

public class TrackBarPropertyGridItem : PropertyGridItemElement
{
    protected override PropertyGridValueElement CreatePropertyGridValueElement()
    {
        return new CustomPropertyGridValueElement(); 
    }
}

public class CustomPropertyGridValueElement : PropertyGridValueElement
{
    RadTrackBarElement _trackbar;

    public RadTrackBarElement Trackbar
    {
        get
        {
            return this._trackbar;
        }
    }

    protected override void CreateChildElements()
    {
        base.CreateChildElements();

        _trackbar = new RadTrackBarElement();
        _trackbar.Minimum = 0;
        _trackbar.Maximum = 100;
       
        this.DrawText = false;
        this.Children.Add(_trackbar);
    }
}



When resizing the form, the applications hangs.

Workaround:

/// <summary>
    /// Track bar property grid element
    /// </summary>
    public class TrackBarPropertyGridItem : PropertyGridItemElement
    {
        class MyTrackBarElement : RadTrackBarElement
        {
            protected override void OnNotifyPropertyChanged(string propertyName)
            {
                if (propertyName == "TickOffSet" || propertyName == "ThumbSize")
                {
                    this.BodyElement.ScaleContainerElement.InvalidateMeasure();
                    this.BodyElement.IndicatorContainerElement.InvalidateMeasure();
                    return;
                }

                base.OnNotifyPropertyChanged(propertyName);
            }

            protected override Type ThemeEffectiveType
            {
                get
                {
                    return typeof(RadTrackBarElement);
                }
            }
        }

        /// <summary>
        /// The trackbar element to be displayed in the cell
        /// </summary>
        private RadTrackBarElement _trackbar;

        /// <summary>
        /// Accessor to the track bar element
        /// </summary>
        public RadTrackBarElement Trackbar
        {
            get { return _trackbar; }
        }

        /// <summary>
        /// Create child elements of the propertyGridItem
        /// </summary>
        protected override void CreateChildElements()
        {
            base.CreateChildElements();

            _trackbar = new MyTrackBarElement();
            _trackbar.Minimum = 0;
            _trackbar.Maximum = 100;
            _trackbar.ShowTicks = false;


            this.ValueElement.DrawText = false;
            this.ValueElement.Children.Add(this._trackbar);
        }

        /// <summary>
        /// Synchronise the value with property grid item
        /// </summary>
        public override void Synchronize()
        {
            base.Synchronize();

            PropertyGridItem item = this.Data as PropertyGridItem;

            this._trackbar.Value = (int)item.Value;
        }


        /// <summary>
        /// Add editor override
        /// </summary>
        /// <param name="editor">input editor</param>
        public override void AddEditor(IInputEditor editor)
        { }

        /// <summary>
        /// Remove editor override
        /// </summary>
        /// <param name="editor">input editor</param>
        public override void RemoveEditor(IInputEditor editor)
        { }

        /// <summary>
        /// Check if the item is comptable with trackbar editor
        /// </summary>
        /// <param name="data">item to check</param>
        /// <param name="context">context</param>
        /// <returns>true if compatible, false otherwise</returns>
        public override bool IsCompatible(PropertyGridItemBase data, object context)
        {
            PropertyGridItem item = data as PropertyGridItem;
            return (item != null && item.PropertyType == typeof(int));
        }

        /// <summary>
        /// Get the type of the property grid element
        /// </summary>
        protected override Type ThemeEffectiveType
        {
            get { return typeof(PropertyGridItemElement); }
        }

To reproduce:

Follow this article http://www.telerik.com/help/winforms/propertygrid-features-custom-grouping.html . You will notice that the CustomGrouping event will not fire.

1. Drag a new RadPropertyGrid to a form.
2. Set the selected object property to any object
3. Add an event handler for the PropertyValueChanged event and add code in it that sets the same object as selected object of the property grid.
4. You will get a NullReferenceException

Please refer to the attached video illustrating the incorrect behavior of the context menu.

Workaround:

        public RadForm1()
        {
            InitializeComponent();

            this.radPropertyGrid1.SelectedObject = this;
            this.radPropertyGrid1.ToolbarVisible = true;

            this.radPropertyGrid1.RadContextMenu = new CustomPropertyGridDefaultContextMenu(this.radPropertyGrid1.PropertyGridElement.PropertyTableElement);
        }

        public class CustomPropertyGridDefaultContextMenu : Telerik.WinControls.UI.PropertyGridDefaultContextMenu
        {
            PropertyGridTableElement tableElement;

            public CustomPropertyGridDefaultContextMenu(PropertyGridTableElement propertyGridElement) : base(propertyGridElement)
            {
                tableElement = propertyGridElement;
            }

            protected override void OnDropDownOpening(CancelEventArgs args)
            {
                base.OnDropDownOpening(args);
                PropertyGridItemBase item = this.tableElement.SelectedGridItem;
                if (item != null)
                {
                    if (!(item is PropertyGridGroupItem))
                    {
                        this.EditMenuItem.Visibility = ElementVisibility.Visible;
                        this.ResetMenuItem.Visibility = ElementVisibility.Visible;
                    }
                }
            }
        }

To reproduce:
  public Form1()
        {
            InitializeComponent();

            PropertyStoreItem intItem = new PropertyStoreItem(typeof(int), "Integer", 1);
            PropertyStoreItem floatItem = new PropertyStoreItem(typeof(float), "Float", 1f, "Property storing a floating point value.");
            PropertyStoreItem stringItem = new PropertyStoreItem(typeof(string), "String", "telerik", "Property storing a string value", "Telerik");
            PropertyStoreItem dockItem = new PropertyStoreItem(typeof(DockStyle), "Dock", DockStyle.Top, "Property containing DockStyle value", "Layout", false);

            RadPropertyStore store = new RadPropertyStore();
            store.Add(intItem);
            store.Add(floatItem);
            store.Add(stringItem);
            store.Add(dockItem);

            this.radPropertyGrid1.SelectedObject = store;
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            PropertyStoreItem intItem = new PropertyStoreItem(typeof(int), "Integer111", 1);
            PropertyStoreItem floatItem = new PropertyStoreItem(typeof(float), "Float111", 1f, "Property storing a floating point value.111");
            PropertyStoreItem stringItem = new PropertyStoreItem(typeof(string), "String111", "telerik", "Property storing a string value", "Telerik111");
            PropertyStoreItem dockItem = new PropertyStoreItem(typeof(DockStyle), "Dock111", DockStyle.Top, "Property containing DockStyle value", "Layout111", false);

            RadPropertyStore store = new RadPropertyStore();
            store.Add(intItem);
            store.Add(floatItem);
            store.Add(stringItem);
            store.Add(dockItem);

            this.radPropertyGrid1.SelectedObject = store;

        }

WORKAOUND: 

radPropertyGrid1.PropertyGridElement.SplitElement.HelpElement.TitleText = "";
radPropertyGrid1.PropertyGridElement.SplitElement.HelpElement.ContentText = "";

To reproduce: use the following code snippet and refer to the attached gif file.

public Form1()
{
    InitializeComponent();
    this.radPropertyGrid1.SelectedObject = this;
    this.radPropertyGrid1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
}

Workaround:

public Form1()
{
    InitializeComponent();
    this.radPropertyGrid1.CreateItemElement += radPropertyGrid1_CreateItemElement;
    this.radPropertyGrid1.SelectedObject = this;
    this.radPropertyGrid1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
}

private void radPropertyGrid1_CreateItemElement(object sender, CreatePropertyGridItemElementEventArgs e)
{
    if (e.ItemElementType == typeof(PropertyGridItemElement))
    {
        e.ItemElementType = typeof(CustomPropertyGridItemElement);
    }
}

public class CustomPropertyGridItemElement : PropertyGridItemElement
{
    private bool isResizing;
    private Point downLocation;
    private int downWidth;

    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(PropertyGridItemElement);
        }
    }

    private const int resizePointerOffset = 3;

    public override bool IsInResizeLocation(Point location)
    {
        return (location.X >= this.ControlBoundingRectangle.X + this.PropertyTableElement.ValueColumnWidth - resizePointerOffset &&
                location.X <= this.ControlBoundingRectangle.X + this.PropertyTableElement.ValueColumnWidth + resizePointerOffset);
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (IsInResizeLocation(e.Location))
        {
            if (this.PropertyTableElement.IsEditing)
            {
                this.PropertyTableElement.EndEdit();
            }

            this.Capture = true;
            this.isResizing = true;
            this.downLocation = e.Location;
            this.downWidth = this.PropertyTableElement.ValueColumnWidth;
        }

        base.OnMouseDown(e);
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        if (this.isResizing)
        {
            int delta = e.Location.X - downLocation.X;

            if (this.RightToLeft)
            {
                delta *= -1;
            }

            this.PropertyTableElement.ValueColumnWidth = downWidth - delta;
            return;
        }

        if (this.IsInResizeLocation(e.Location))
        {
            this.ElementTree.Control.Cursor = Cursors.VSplit;
        }
        else
        {
            this.ElementTree.Control.Cursor = Cursors.Default;
        }

        base.OnMouseMove(e);
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        if (this.isResizing)
        {
            this.isResizing = false;
            this.Capture = false;
        }

        base.OnMouseUp(e);
    }
}

To reproduce:
public Form1()
{
    InitializeComponent();
    this.radPropertyGrid1.Size = new System.Drawing.Size(272, 135);
    PropertyStoreItem intItem = new PropertyStoreItem(typeof(int), "Integer", 1);
    PropertyStoreItem showTrend = new PropertyStoreItem(typeof(bool), "ShowTrend", 0);
    PropertyStoreItem trendItem1 = new PropertyStoreItem(typeof(int), "TrendTypes", 0);
    // Case of Re-loading the previously Saved Trend Type
    PropertyStoreItem floatItem = new PropertyStoreItem(typeof(float), "Float", 1f,
        "Property storing a floating point value.");
    PropertyStoreItem stringItem = new PropertyStoreItem(typeof(string), "String", "Telerik",
        "Property storing a string value", "Telerik");
    PropertyStoreItem dockItem = new PropertyStoreItem(typeof(DockStyle), "Dock", DockStyle.Top,
        "Property containing DockStyle value", "Layout", false);
    RadPropertyStore store = new RadPropertyStore();
    store.Add(intItem);
    store.Add(showTrend);
    store.Add(trendItem1);
    store.Add(floatItem);
    store.Add(stringItem);
    store.Add(dockItem);
    this.radPropertyGrid1.SelectedObject = store;
   radPropertyGrid1.Items["TrendTypes"].Visible = false;
    this.radPropertyGrid1.PropertyValueChanged += radPropertyGrid1_ValueChanged;

}

private void radPropertyGrid1_ValueChanged(object sender, PropertyGridItemValueChangedEventArgs e)
{

    PropertyGridItem current = (PropertyGridItem)e.Item;
    if (current.Name == "ShowTrend")
    {
        bool val = Convert.ToBoolean(current.Value);
        radPropertyGrid1.Items["TrendTypes"].Visible = val;
    }

}

- There is no scrollbar when the item is shown.

Workaround:
radPropertyGrid1.PropertyGridElement.PropertyTableElement.Update(PropertyGridTableElement.UpdateActions.ExpandedChanged);

Please refer to the attached gif file demonstrating how to reproduce the error with the Demo application.

Workaround: change the DropDownStyle property to DropDownList:

private void radPropertyGrid1_EditorInitialized(object sender, PropertyGridItemEditorInitializedEventArgs e)
{
    PropertyGridDropDownListEditor editor = e.Editor as PropertyGridDropDownListEditor;
    if (editor!=null)
    {
        BaseDropDownListEditorElement editorElement = editor.EditorElement as BaseDropDownListEditorElement;
        editorElement.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDownList;
    }
}

To reproduce: use the following code:

Items _items = new Items();

public Form1()
{
    InitializeComponent(); 

    radPropertyGrid1.SelectedObject = _items;
}

class Items
{
    [Description("A Bool Property")]
    public bool IsEnabled
    {
        get
        {
            return _isEnabled;
        }
        set
        {
            _isEnabled = value;
            RadMessageBox.Show("IsEnabled");
        }
    }
    private bool _isEnabled;

    [Description("A String Property")]
    public string Name
    {
        get { return _name; }
        set
        {
            _name = value;
            RadMessageBox.Show("Name");
        }
    }
    private string _name ;

}

Steps to perform:
1. Change the "Name" property and leave the editor active.
2. Click on the check box to change the "IsEnabled" property.

As a result you will notice that the "IsEnabled" setter is called before the "Name" setter.

Workaround:

Items _items = new Items();

public Form1()
{
    InitializeComponent(); 

    this.radPropertyGrid1.CreateItemElement += radPropertyGrid1_CreateItemElement;        
    radPropertyGrid1.SelectedObject = _items;
}

private void radPropertyGrid1_CreateItemElement(object sender, CreatePropertyGridItemElementEventArgs e)
{
    if (e.ItemElementType == typeof(PropertyGridCheckBoxItemElement))
    {
        e.ItemElementType = typeof(CustomPropertyGridCheckBoxItemElement);
    }
}

public class CustomPropertyGridCheckBoxItemElement : PropertyGridCheckBoxItemElement
{
    public CustomPropertyGridCheckBoxItemElement()
    {
        this.CheckBoxElement.ToggleStateChanging += CheckBoxElement_ToggleStateChanging;
    }

    private void CheckBoxElement_ToggleStateChanging(object sender, StateChangingEventArgs args)
    {
        if (this.PropertyTableElement.ActiveEditor != null && this.PropertyTableElement.SelectedGridItem != this.Data)
        {
            this.PropertyTableElement.EndEdit();
        }
    }

    protected override void DisposeManagedResources()
    {
        this.CheckBoxElement.ToggleStateChanging -= CheckBoxElement_ToggleStateChanging;

        base.DisposeManagedResources();
    }
}

To reproduce:
- Add a property grid to a blank form set the selected object the the same form.
- Group the items and click several times to expand collapse the groups or items.
- you will notice that some of expanded/collapsed groups icon is in wrong state (same applies for the +/- sign of the properties).

Please refer to the attached sample project.