Workaround:
public Form1()
{
    InitializeComponent(););

    this.richTextBox1.Text = "thess is is a tesssset";

    this.radSpellChecker1.AutoSpellCheckControl = this.richTextBox1;
    this.radSpellChecker1.SpellingFormShowing += radSpellChecker1_SpellingFormShowing;
}

private void radSpellChecker1_SpellingFormShowing(object sender, SpellingFormShowingEventArgs e)
{
    e.SpellingForm.FormClosed -= SpellingForm_FormClosed;
    e.SpellingForm.FormClosed += SpellingForm_FormClosed;
}

private void SpellingForm_FormClosed(object sender, FormClosedEventArgs e)
{
    this.radSpellChecker1.AutoSpellCheckControl = null;
    this.radSpellChecker1.AutoSpellCheckControl = this.richTextBox1;
}

How to reproduce:
 public RadForm1()
 {
     InitializeComponent();

     this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
     this.radDateTimePicker1.CustomFormat = "MM/dd/yyyy";
     this.radDateTimePicker1.DateTimePickerElement.Calendar.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Zoom;
     ((Telerik.WinControls.UI.MaskDateTimeProvider)this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.Provider).AutoSelectNextPart = true;
 }

Workaround:
 public RadForm1()
 {
     InitializeComponent();

     this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
     this.radDateTimePicker1.CustomFormat = "MM/dd/yyyy";
     this.radDateTimePicker1.DateTimePickerElement.Calendar.HeaderNavigationMode = Telerik.WinControls.UI.HeaderNavigationMode.Zoom;
     ((Telerik.WinControls.UI.MaskDateTimeProvider)this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.Provider).AutoSelectNextPart = true;

     this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.KeyDown += TextBoxElement_KeyDown;

 }

 private void TextBoxElement_KeyDown(object sender, KeyEventArgs e)
 {
     RadMaskedEditBoxElement tb = (RadMaskedEditBoxElement)sender;
     MaskDateTimeProvider provider = (MaskDateTimeProvider)tb.Provider;

     switch (provider.List[provider.SelectedItemIndex].type)
     {
         case PartTypes.Month:
             if (e.KeyCode == Keys.Left)
             {
                 provider.SelectedItemIndex = provider.List.Count;
             }

             break;
         case PartTypes.Year:
             if (e.KeyCode == Keys.Right)
             {
                 provider.SelectedItemIndex = -1;
             }
             break;
     }
 }

To reproduce:
-Add RadBrowseEditor to a form and set the DialogType to FolderBrowseDialog.
-At runtime click the browse button to open the "Browse For Folder" dialog and click the "Make New Folder", type a name for the newly created folder BUT DO NOT PRESS THE ENTER KEY and click the OK button, the Value is set to "New Folder" instead of the name I have typed.

RadMaskedEditBox - has incorrect behavior when decimal separator is "D". You can set only one digit before separator.

RadMaskedEditBox - has incorrect behavior if you set values with differed length.

Workaround is to reset the value before to set new one:

C#
RadMaskedEditBox1.Value = null;
RadMaskedEditBox1.Value = "YourText";

VB
RadMaskedEditBox1.Value = Nothing
RadMaskedEditBox1.Value = "YourText"

The initial loading performance can be improved. 

The performance can be improved by creating the RadContextMenu only when it is needed not when the element is created.

All controls (i.e. RadGridView, RadPropertyGrid, etc.), which use textbox editor, containing HostedTextBoxBase descendant, can be spelled by the RadSpellChecker. But the red underline is not displayed entirely. The inside hosted text box should have a bigger height.

To reproduce: use the following code:
public Form1()
{
    InitializeComponent();
    
    RadMaskedEditBox maskControl = new RadMaskedEditBox();
    maskControl.TextMaskFormat = System.Windows.Forms.MaskFormat.ExcludePromptAndLiterals;
    CultureInfo ci = CultureInfo.CreateSpecificCulture(Thread.CurrentThread.CurrentCulture.Name);
    ci.NumberFormat.NumberGroupSeparator = "";
    maskControl.Culture = ci;
    maskControl.Text = "2";
    object o = maskControl.Value;  //Exception!! 
    this.Controls.Add(maskControl);
}

Workaround:
Do not set NumberGroupSeparator to String.Empty when TextMaskFormat=ExcludePromptAndLiterals

To reproduce: 

this.radDateTimePicker1.Format = DateTimePickerFormat.Custom;
 this.radDateTimePicker1.CustomFormat = "MM/yyyy";
this.radDateTimePicker1.Value = new DateTime(2016, 01, 31);

Select the month part and try to change it to 02.

Workaround: initialize with the first day of the month.

To reproduce:
this.radTextBox1.Text = "sample";
this.radTextBox1.TextBoxElement.BackColor = Color.Red;
this.radTextBox1.TextBoxElement.UseDefaultDisabledPaint = false;
this.radTextBox1.TextBoxElement.Enabled = false;

In previous version (e.g. Q1 2014 SP1) the TextBoxElement is not rendered red.

1. Add a RadMaskedEditBox and a RadCheckbox to your form.
2. Add ToggleStateChanged event to the RadCheckbox.
3. Use the following code:
private void radCheckBox1_ToggleStateChanged(object sender, StateChangedEventArgs args)
{
    if (args.ToggleState == Telerik.WinControls.Enumerations.ToggleState.On)
    {
        radMaskedEditBox1.Mask = "^[0-9]{5}$";
        radMaskedEditBox1.MaskType = MaskType.Regex;
    }
 
    else if (args.ToggleState == Telerik.WinControls.Enumerations.ToggleState.Off)
    {
        radMaskedEditBox1.Mask = "";
        radMaskedEditBox1.MaskType = MaskType.None;
    }
}
4. Start the application
5. Check and uncheck the checkbox (now ToggleStateChanged event was fired)
6. Type some letters to the RadMaskedEditBox (e.g. "aaa")
7. Press tab

Workaround:
RegexMaskTextBoxProvider provider;
private void radCheckBox1_ToggleStateChanged(object sender, Telerik.WinControls.UI.StateChangedEventArgs args)
{
    if (args.ToggleState == Telerik.WinControls.Enumerations.ToggleState.On)
    {
        radMaskedEditBox1 = new RadMaskedEditBox();
        radMaskedEditBox1.Mask = "^[0-9]{5}$";
        radMaskedEditBox1.MaskType = MaskType.Regex;
        radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.HostedControl.Leave += HostedControl_Leave;
        provider = (RegexMaskTextBoxProvider) radMaskedEditBox1.MaskedEditBoxElement.Provider;
    }

    else if (args.ToggleState == Telerik.WinControls.Enumerations.ToggleState.Off)
    {
        radMaskedEditBox1.MaskedEditBoxElement.Mask = "";
        radMaskedEditBox1.MaskType = MaskType.None;
    }
}

private void HostedControl_Leave(object sender, EventArgs e)
{
    if (radMaskedEditBox1.MaskType == MaskType.None && provider != null)
    {
        provider.GetType().GetField("mask", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).SetValue(provider, "*");
    }
}

To reproduce:
- Set the mask like this:
private void RadForm1_Load(object sender, EventArgs e)
{
    radMaskedEditBox1.MaskType = MaskType.Regex;
    radMaskedEditBox1.Mask = "^[0-9]{6} [A-Z]{4} [0-9]{7}$|^[0-9]{5}$";
}
- Start the application, enter five digits and pres Tab.
- The error icon apperas.

Workaround:
- Set the Mask before the MaskType:
radMaskedEditBox1.Mask = "^[0-9]{6} [A-Z]{4} [0-9]{7}$|^[0-9]{5}$";
radMaskedEditBox1.MaskType = MaskType.Regex;

To reproduce: 

Populate the AutoCompleteBox and start a search returning enough results to need the scrollbar. When clicking and dragging the scroll bar to scroll - if the cursor is not positioned directly over the scrollbar at the time you release the mouseclick, the dropdown list is closed an item is added to the selection.

Workaround: 
 public partial class Form1 : Form
    {
        bool isMouseDown = false;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            AddAutoCompleteItems();
            radAutoCompleteBox1.ListElement.VScrollBar.MouseDown += ListElement_MouseDown;
            radAutoCompleteBox1.ListElement.VScrollBar.MouseUp += ListElement_MouseUp;
            radAutoCompleteBox1.TokenValidating += radAutoCompleteBox1_TokenValidating;
        }

        void radAutoCompleteBox1_TokenValidating(object sender, TokenValidatingEventArgs e)
        {
            e.IsValidToken = !isMouseDown;
            isMouseDown = false;
        }

        void ListElement_MouseUp(object sender, MouseEventArgs e)
        {
            RadListVisualItem el = radAutoCompleteBox1.ListElement.ElementTree.GetElementAtPoint(e.Location) as RadListVisualItem;

            if (el != null)
            {
                return;
            }
            
            isMouseDown = false;
        }

        void ListElement_MouseDown(object sender, MouseEventArgs e)
        {
            isMouseDown = true;
        }

        private void AddAutoCompleteItems()
        {
            RadListDataItemCollection items = this.radAutoCompleteBox1.AutoCompleteItems;

            items.Add(new RadListDataItem("Joe Smith", "joe@fakecompany.com"));
            items.Add(new RadListDataItem("Adam Petersen", "adam@qwerty.com"));
            items.Add(new RadListDataItem("Jack Russel", "jack@russel.nocom"));
            items.Add(new RadListDataItem("Daniel Finger", "daniel.pinger@gmail.com"));
            items.Add(new RadListDataItem("Richard Vail", "rvail@richardvail.com"));
            items.Add(new RadListDataItem("Sebastian Jonnson", "s.jonnson@sjonnson.com"));
            items.Add(new RadListDataItem("Lee Cooper", "lee.cooper@coopercoorp.com"));
            items.Add(new RadListDataItem("Kelvin Clain", "kclain@clainkevin.com"));
            items.Add(new RadListDataItem("Maria Jenson", "mjenson@mariajenson.com"));
            items.Add(new RadListDataItem("Chelsea Maarten", "chelsea@maarten.com"));
            items.Add(new RadListDataItem("Jenson Chew", "jenson.chew@nospam.com"));
            items.Add(new RadListDataItem("Martin Williams", "m.williams@martinandwilliams.com"));
            items.Add(new RadListDataItem("Telerik", "clientservice@telerik.com"));
            items.Add(new RadListDataItem("James Stone", "james.stone@manystones.com"));
            items.Add(new RadListDataItem("Samuel Jackson", "samuel.jackson@nojackson.com"));
        }
    }

    public class MyRadAutoCompleteBox : RadAutoCompleteBox
    {
        protected override RadTextBoxControlElement CreateTextBoxElement()
        {
            return new MyRadAutoCompleteBoxElement();
        }
    }

    public class MyRadAutoCompleteBoxElement : RadAutoCompleteBoxElement
    {
        protected override TextBoxViewElement CreateViewElement()
        {
            return new MyAutoCompleteBoxViewElement();
        }

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

    public class MyAutoCompleteBoxViewElement : AutoCompleteBoxViewElement
    {
        protected override int InsertTokenizedTextBlocks(int index, string text, bool performInvalidation)
        {
            bool isValid = performInvalidation ? this.OnTokenValidating(text) : true;

            if (!isValid)
            {
                return index;
            }
            return base.InsertTokenizedTextBlocks(index, text, performInvalidation);
        }

To reproduce:
- Select the time from the drop down and press close button.
- Notice that the form is deactivated.

Workaround:
void radTimePicker1_LostFocus(object sender, EventArgs e)
{
    this.Activate();
}

To reproduce:
this.radTextBoxControl1.Text = "dog and blue glue";
this.radTextBoxControl1.Size = new Size(50, 65);
this.radTextBoxControl1.Multiline = true;

Workaround:
use RadTextBox.

Multiply line text is not correct align at the left side when control runs on Win xp.

Steps to reproduce:
this.radTextBoxControl1.Text = "This\nis a\ntest";

To reproduce:

1. Please refer to the attached sample project.
2. Run the project and click the button. The two RadTextBox controls are left aligned.
3. Close the opened form and click the button again. You will notice that the RadTextBox is not left aligned.

Workaround: use RadTextBoxControl.

To reproduce:
- Add RadtextBox to a form and anchor it to the left and right.
- Set WindosState of the form to Maximized
- Start the application

Workaround
void RadForm1_Shown(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Maximized;
}

To reproduce:
public RadForm1()
{
    InitializeComponent();

    this.radTextBoxControl1.Text = "Sample text with misspelledd word";
    RadSpellChecker spellChecker1 = new RadSpellChecker();
    spellChecker1.AutoSpellCheckControl = radTextBoxControl1;
}

private void radTextBoxControl1_ContextMenuOpening(object sender, TreeBoxContextMenuOpeningEventArgs e)
{
    RadMenuItem customItem= new RadMenuItem("Custom item");
    e.ContextMenu.Items.Add(customItem);
}

Workaround: use RadTextBox instead:
RadSpellChecker spellChecker1 = new RadSpellChecker();    
this.radTextBox1.Text = "Sample text with misspelledd word";
spellChecker1.AutoSpellCheckControl = this.radTextBox1;
TextBoxSpellChecker tbSpellChecker = spellChecker1.GetControlSpellChecker(typeof(RadTextBox)) as TextBoxSpellChecker;
tbSpellChecker.DropDownMenu.DropDownOpening += DropDownMenu_DropDownOpening;

private void DropDownMenu_DropDownOpening(object sender, CancelEventArgs e)
{
    RadDropDownMenu menu = sender as RadDropDownMenu;
    RadMenuItem customItem = new RadMenuItem("Custom item");
    menu.Items.Add(customItem);
}