To reproduce:
public partial class Form1 : Form
{
	public Form1()
	{
		InitializeComponent();
        var data = new List<Thing>();
        this.radGridView1.DataSource = data ;
        this.radGridView1.DataError += RadGridView1_DataError;
	}

    private void RadGridView1_DataError(object sender, Telerik.WinControls.UI.GridViewDataErrorEventArgs e)
    {
       
    }

    public class Thing
	{
		private Thing() { }
	}
}

To reproduce: 
1.Set the GridView template's ViewDefinition to ColumnGroups
2.Add a new column
3.Delete the column
4.Add another new column. The PropertyBuilder closes and it is not possible to open the Smart Tag of RadGridView anymore. Please refer to the attached gif file.

Workaround:
The issue is caused by a remaining reference to a deleted column. When using the RadGridView PropertyBuilder and a column is added to ColumnGroupsViewDefinition, once the column is deleted from RadGridView (via the Property Builder) the code for adding the column in the ColumnGroupsViewDefinition remains, and this causes further designer instability with the Property Builder.

To handle this scenario there are two options:
1. When a column is deleted via Property Builder, and this column was part of ColumnGroupsViewDefinition, open the Designer.cs/vb file, locate the line that adds this column to a ColumnNames collection and delete it. It should be something like:
gridViewColumnGroupRow3.ColumnNames.Add("column1"); - where "column1" is the name of the deleted column

2. Instead of using the Property Builder to build the ColumnGroupsViewDefinition, build it programmatically.

To reproduce:
- Sort the grid and then reset the data source.
- Format the rows like this:
void radGridView1_RowFormatting(object sender, Telerik.WinControls.UI.RowFormattingEventArgs e)
{
    string exception_status = "";
    string completed_user = "";

    if (!DBNull.Value.Equals(e.RowElement.RowInfo.Cells["Name"].Value))
    {
        exception_status = e.RowElement.RowInfo.Cells["Name"].Value.ToString();
    }

    if (!DBNull.Value.Equals(e.RowElement.RowInfo.Cells["Drug"].Value))
    {
        completed_user = e.RowElement.RowInfo.Cells["Drug"].Value.ToString().Trim();
    }

    if (exception_status == "Sam" && completed_user == "Enebrel")
    {
        e.RowElement.ForeColor = Color.Red;
    }
    else
    {
        e.RowElement.ForeColor = Color.Black;
    }
}
Workaround:
if (e.RowElement.RowInfo.Cells.Count ==0)
{
    return;
}

Similar property is already available in the property grid and tree view exporters.

To reproduce:

BindingList<Item> items = new BindingList<Item>();

public Form1()
{
    InitializeComponent();
    
    for (int i = 0; i < 5; i++)
    {
        items.Add(new Item(i,i * 1.25m));
    }
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

    GridViewSummaryItem summaryItem = new GridViewSummaryItem();
    summaryItem.Name = "Price";
    summaryItem.Aggregate = GridAggregateFunction.Sum;
    GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
    summaryRowItem.Add(summaryItem);
    this.radGridView1.SummaryRowsTop.Add(summaryRowItem);
    this.timer1.Start();
}

public class Item  
{
    private int id;
    private decimal price;

    public Item(int id, decimal price)
    {
        this.id = id;
        this.price = price;
    }

    public int Id
    {
        get
        {
            return this.id;
        }
        set
        {
            this.id = value;
        }
    }

    public decimal Price
    {
        get
        {
            return this.price;
        }
        set
        {
            this.price = value; 
        }
    }
}

private void timer1_Tick(object sender, EventArgs e)
{
    items[0].Price += 1;
    this.radGridView1.Rows[0].InvalidateRow();
    this.radGridView1.MasterView.SummaryRows[0].InvalidateRow(); 
}

Workaround: refresh the MasterTemplate or implement the INotifyPropertyChanged

Work around: create a custom GridCheckBoxHeaderCellElement and override the SetCheckBoxState method

public class MyGridCheckBoxHeaderCellElement : GridCheckBoxHeaderCellElement
{
    public MyGridCheckBoxHeaderCellElement(GridViewColumn column, GridRowElement row)
        : base(column, row)
    {
    }

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

    protected override bool SetCheckBoxState()
    {
        bool foundNonCheckedRows = false;
        foreach (GridViewRowInfo row in this.ViewInfo.Rows)
        {
            foundNonCheckedRows = this.CheckCheckBoxValueForAllRows(row, foundNonCheckedRows);
            if (foundNonCheckedRows)
            {
                SetCheckBoxState(ToggleState.Off);
                break;
            }
        }

        if (!foundNonCheckedRows && this.ViewInfo.Rows.Count > 0) //we must have at least one row in order to check the header cell
        {
            SetCheckBoxState(ToggleState.On);
        }

        return foundNonCheckedRows;
    }

    private bool CheckCheckBoxValueForAllRows(GridViewRowInfo row, bool foundNonCheckedRows)
    {
        GridViewChildRowCollection currentRowColection;
        if (foundNonCheckedRows)
        {
            return foundNonCheckedRows;
        }


        if (row.HasChildRows())
        {
            currentRowColection = row.ChildRows;
        }
        else
        {
            currentRowColection = row.ViewInfo.Rows;
        }

        foreach (GridViewRowInfo rowInfo in currentRowColection)
        {
            GridViewGroupRowInfo groupInfo = rowInfo as GridViewGroupRowInfo;
            if (groupInfo != null)
            {
                foundNonCheckedRows = this.CheckCheckBoxValueForAllRows(groupInfo, foundNonCheckedRows);
            }
            else
            {
                object cellValue = rowInfo.Cells[this.ColumnIndex].Value;
                bool? valueFromColumnTypeConvertor = null;
                if (this.ColumnInfo is GridViewDataColumn)
                {
                    TypeConverter convertor = ((GridViewDataColumn)this.ColumnInfo).DataTypeConverter;
                    if (convertor != null && (convertor.CanConvertTo(typeof(bool)) || convertor.CanConvertTo(typeof(ToggleState))))
                    {
                        try
                        {
                            valueFromColumnTypeConvertor = Convert.ToBoolean(convertor.ConvertTo(cellValue, typeof(bool)));
                        }
                        catch (FormatException)//convert to bool fails
                        {
                        }
                        catch (NotSupportedException)//coverting NULL
                        {
                        }
                    }
                }

                try
                {
                    if (rowInfo != this.RowInfo &&
                        (cellValue == null || cellValue == DBNull.Value) ||
                        (valueFromColumnTypeConvertor.HasValue && !valueFromColumnTypeConvertor.Value) ||
                        (!valueFromColumnTypeConvertor.HasValue && !Convert.ToBoolean(cellValue)))
                    {
                        foundNonCheckedRows = true;
                    }
                }
                catch (FormatException)//convert to bool fails
                {
                }
            }
        }

        return foundNonCheckedRows;
    }
}

To reproduce:
- Add 100000 rows to the grid.
- Filter the grid so there are no rows.
- Call the BestFitColumns method:
radGridView1.MasterTemplate.BestFitColumns(Telerik.WinControls.UI.BestFitColumnMode.DisplayedCells)

Workaround:
If gridview.MasterTemplate.DataView.Count > 0 Then
	gridview.MasterTemplate.BestFitColumns()
End If

To reproduce:

public Form1()
{
    InitializeComponent();
    DataTable dt = new DataTable();
    for (int i = 0; i < 10; i++)
    {
        dt.Columns.Add("Col" + i);
    }

    for (int i = 0; i < 10; i++)
    {
        DataRow row = dt.NewRow();
        foreach (DataColumn col in dt.Columns)
        {
            row[col.ColumnName] = "Data " + i + "." + dt.Columns.IndexOf(col);
        }
        dt.Rows.Add(row);
    }
    this.radGridView1.DataSource = dt;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

    this.radGridView1.AddNewRowPosition = Telerik.WinControls.UI.SystemRowPosition.Bottom;
    this.radGridView1.Columns[9].IsVisible = false;
    this.radGridView1.Columns[8].ReadOnly = true;

    this.radGridView1.Columns[2].IsVisible = false;
    this.radGridView1.Columns[4].IsVisible = false;
    this.radGridView1.Columns[6].IsVisible = false;
}


Workaround: use a custom row behavior and control what to be the selected cell when pressing the Tab key for example: 

public Form1()
{
    InitializeComponent();
    DataTable dt = new DataTable();
    for (int i = 0; i < 10; i++)
    {
        dt.Columns.Add("Col" + i);
    }

    for (int i = 0; i < 10; i++)
    {
        DataRow row = dt.NewRow();
        foreach (DataColumn col in dt.Columns)
        {
            row[col.ColumnName] = "Data " + i + "." + dt.Columns.IndexOf(col);
        }
        dt.Rows.Add(row);
    }
    this.radGridView1.DataSource = dt;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

    this.radGridView1.AddNewRowPosition = Telerik.WinControls.UI.SystemRowPosition.Bottom;
    this.radGridView1.Columns[9].IsVisible = false;
    this.radGridView1.Columns[8].ReadOnly = true;

    this.radGridView1.Columns[2].IsVisible = false;
    this.radGridView1.Columns[4].IsVisible = false;
    this.radGridView1.Columns[6].IsVisible = false;

    //register the custom row  behavior
    BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
    gridBehavior.UnregisterBehavior(typeof(GridViewNewRowInfo));
    gridBehavior.RegisterBehavior(typeof(GridViewNewRowInfo), new CustomGridHierarchyRowBehavior());
}

public class CustomGridHierarchyRowBehavior : GridNewRowBehavior
{
    protected override bool ProcessTabKey(KeyEventArgs keys)
    {
        bool isLastColumn = this.Navigator.IsLastColumn(this.GridControl.CurrentColumn);
        bool result = base.ProcessTabKey(keys);
        if (isLastColumn)
        {
            this.GridControl.CurrentColumn = this.GridControl.Columns[0];
            this.GridControl.BeginEdit();
        }
        return result;
    }
}

To reproduce:
ColumnGroupsViewDefinition view = new ColumnGroupsViewDefinition();
view.ColumnGroups.Add(new GridViewColumnGroup("Customer Contact"));

view.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
view.ColumnGroups[0].Rows[0].ColumnNames.Add("Address");
view.ColumnGroups[0].Rows[0].ColumnNames.Add("Contact");

radGridView1.Columns.Add("Address");
radGridView1.Columns.Add("Contact");

radGridView1.Columns[0].Width = 300;
radGridView1.Columns[1].Width = 300;
radGridView1.ViewDefinition = view;

for (int i = 0; i < 10; i++)
{
    radGridView1.Rows.Add("test test test test test", "test");
}

Workaround:
docs.telerik.com/devtools/winforms/telerik-presentation-framework/export-renderers/spreadexportrenderer

To reproduce:
- Add date time column to a grid and set the MaskType to FreeFormDateTime (this can be done by accessing the editor in the CellEditorInitialized event).
- Type a year in the editor and click the right arrow key until the cell is changed.

Workaround:
class MyRadDateTimeEditor : RadDateTimeEditor
{
    bool isRightmostMaskItemSelected = false;

    public override void OnKeyDown(KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Right)
        {
            FreeFormDateTimeProvider provider = ((RadDateTimeEditorElement) this.EditorElement).TextBoxElement.Provider as FreeFormDateTimeProvider; 
            if (provider.TextBoxItem.SelectionStart == provider.TextBoxItem.Text.Length)
            {
                this.isRightmostMaskItemSelected = true;
            }
        }
        else
        {
            base.OnKeyDown(e);
        }
    }
    protected override void OnKeyUp(KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Right)
        {
            if (isRightmostMaskItemSelected)
            {
                isRightmostMaskItemSelected = false;
                GridDateTimeCellElement cell = this.OwnerElement as GridDateTimeCellElement;
                cell.GridControl.EndEdit();
            }
        }
        else
        {
            base.OnKeyUp(e);
        }
    }
}

void radGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
{
    RadDateTimeEditor editor = e.ActiveEditor as RadDateTimeEditor;
    if (editor != null)
    {
        var editorElement = editor.EditorElement as RadDateTimeEditorElement;
        editorElement.TextBoxElement.MaskType = MaskType.FreeFormDateTime;
    }
}

Please find attached a sample project.

1. Select the new row and activate te editor.
2. Do not enter any value and click outside the new row. The exception is thrown.

Workaround: bind directly to the query result:
this.radGridView1.DataSource = (from p in context.Customers select p).ToList();

To reproduce:
-Create a new ColumnGroupsViewDefinition and pin the first group.
- Asign the view to the grid.

Workaround:
Pin the group after the view is asigned.
 

To reproduce: use the attached sample project

Workaround: refresh the template when updating the data source:
this.radGridView1.MasterTemplate.Refresh();

To reproduce:
- Use RowFormatting to change the font style to bold.
- Call the  BestFitColumns method.

To reproduce:
- Add a grid with some columns at design time.
- Open the designer code and you will notice the TableViewDefinition.

Workaround:
Manually delete the TableViewDefinition. 

To reproduce:

Sub New()        
    InitializeComponent() 
    Dim items As New List(Of WorkOrder)
    For index = 1 To 56
        items.Add(New WorkOrder(index, "Item" & index, 0))
    Next
    Dim rand As New Random
    For index = 57 To 500
        items.Add(New WorkOrder(index, "Item" & index, rand.Next(1, 56)))
    Next

    Me.RadGridView1.Relations.AddSelfReference(Me.RadGridView1.MasterTemplate, "Id", "ParentId")
    Me.RadGridView1.DataSource = items
    Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    Me.RadGridView1.EnablePaging = True
    Me.RadGridView1.PageSize = 20
End Sub

Public Class WorkOrder
    Private _id As Integer
    Private _name As String
    Private _parentId As Integer
    
    Public Sub New(id As Integer, name As String, parentId As Integer)
        Me._id = id
        Me._name = name
        Me._parentId = parentId
    End Sub

    Public Property Id() As String
            Get
                Return _id
            End Get
            Set(ByVal value As String)
                _id = value
            End Set
        End Property
    
    Public Property Name() As String
            Get
                Return _name
            End Get
            Set(ByVal value As String)
                _name = value
            End Set
        End Property

    Public Property ParentID() As Integer
            Get
                Return _parentId
            End Get
            Set(ByVal value As Integer)
                _parentId = value
            End Set
        End Property
End Class

Workaround: use standard hierarchy: http://docs.telerik.com/devtools/winforms/gridview/hierarchical-grid/hierarchy-of-one-to-many-relations

Note: as a result ArgumentException is thrown when accessing a cell value.

To reproduce:

private DataTable dt;
private GridViewRowInfo selectedRow;

public Form1()
{
    InitializeComponent();

    PopulateGrid();
    PopulateGrid();
} 

private void PopulateGrid()
{
    dt = new DataTable();
    dt.Columns.Add("Version No", typeof(Int32));

    DataRow dr = dt.NewRow();
    dr["Version No"] = 2385;
    dt.Rows.Add(dr);

    this.radGridView1.DataSource = dt;

    MessageBox.Show(selectedRow.Cells[0].ColumnInfo.Name);
    MessageBox.Show(selectedRow.Cells[0].Value.ToString()); //This will error on the second time through
}

private void radGridView1_CurrentRowChanged(object sender, CurrentRowChangedEventArgs e)
{
    //This event is not firing on the second population of the grid in the new Telerik version.
    //In the old version, it did

    if (!this.radGridView1.CurrentRow.IsSystem && !this.radGridView1.CurrentRow.HasChildRows() && this.radGridView1.CurrentRow.Cells.Count > 0)
        selectedRow = this.radGridView1.CurrentRow;
}

Workaround: use the RadGridView.CurrentRow property directly when accessing the cell value.

To reproduce:
- Add some columns to a grid and make one read-only.
- Add a value to the new row then click the read-only cell.
- Click outside of the grid (focus another control). 
- The new row is not added.

Workaround:
radgridView1.EndEdit()
radgridView1.MasterView.TableAddNewRow.EndAddNewRow()