Attached project for reproducing.

Workaround:
var column = radGridView1.Columns[2] as GridViewDecimalColumn;
var format = column.FormatString;
column.FormatString = "";
column.ExcelExportType = DisplayFormatType.Currency;

//export the grid

column.FormatString = format;

Please refer to the attached sample project and follow the steps illustrated in the attached gif file.

To reproduce:
- Add a grid to a PageView and add the pageView to a dock window at run time.
- The grid should not be visible.
- Set your DPI setting to 150%

Workaround:
class MyViewDefinition : TableViewDefinition
{
    public override IRowView CreateViewUIElement(GridViewInfo viewInfo)
    {
        return new MyTableElement();
    }
}
class MyTableElement : GridTableElement
{
    public override void DpiScaleChanged(SizeF scaleFactor)
    {
        if (this.ViewTemplate != null)
        {
            base.DpiScaleChanged(scaleFactor);
        }

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

//use the above definition like this:

 radGridView1.ViewDefinition = new MyViewDefinition();

To reproduce:
        public Form1()
        {
            InitializeComponent();
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("ParentId", typeof(int));

            for (int i = 0; i < 3; i++)
            {
                dt.Rows.Add(i, "Parent" + i, -1);
            }
            Random rand = new Random();
            for (int i = 3; i < 15; i++)
            {
                dt.Rows.Add(i,"Child"+i,rand.Next(0,3));
            }

            this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "Id", "ParentId");
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.EnableGrouping = true;
            this.radGridView1.EnableCustomGrouping = true;
            this.radGridView1.CustomGrouping+=radGridView1_CustomGrouping;
         }

private void radGridView1_CustomGrouping(object sender, GridViewCustomGroupingEventArgs e)
{
     
}

To reproduce:
1. Bind RadGridView to the Northwind.Customers table.
2. Activate the editor for some of the cells from the CustomerID column.
3. Enter "ALFKI" which already exists. The DataError event is supposed to be fired in this case. If you set the  GridViewDataErrorEventArgs.ThrowException argument to true the exception is supposed to be thrown. However, nothing happens. Please refer to the attached sample project.

How to reproduce check the attached project: 

Workaround: Create a custom spread export renderer

public class MySpreadExportRenderer : SpreadExportRenderer
{
    public override void AddWorksheet(string sheetName)
    {
        string name = "";

        Workbook wb = (Workbook)typeof(SpreadExportRenderer).GetField("workbook", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(this);
        if (sheetName == null || sheetName == string.Empty)
        {
            name = "Sheet_" + (wb.Worksheets.Count + 1);
        }
        else
        {
            name = sheetName;
        }

        if (wb.Worksheets.Where(w => w.Name == name).ToList().Count == 0)
        {
            base.AddWorksheet(sheetName);
        }
        else
        {
            typeof(SpreadExportRenderer).GetField("worksheet", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(this, wb.Worksheets.Add());
        }
    }
}

To reproduce: please refer to the attached gif file. 

Workaround: 

public RadForm1()
{
    InitializeComponent();

    this.radGridView1.CellClick+=radGridView1_CellClick;
}

private void radGridView1_CellClick(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
{
    if (e.Column is Telerik.WinControls.UI.GridViewRowHeaderColumn)
    {
        this.radGridView1.CloseEditor();
    } 
}

Workaround: 
Friend Class MyRadGridView
    Inherits RadGridView

    Public Overrides Property ThemeClassName As String
        Get
            Return GetType(RadGridView).FullName
        End Get
        Set(value As String)
            MyBase.ThemeClassName = value
        End Set
    End Property

    Protected Overrides Function CreateGridViewElement() As RadGridViewElement
        Return New MyRadGridViewElement()
    End Function

End Class

Friend Class MyRadGridViewElement
    Inherits RadGridViewElement

    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(RadGridViewElement)
        End Get
    End Property

    Protected Overrides Function CreateGroupPanelElement() As GroupPanelElement
        Return New MyGroupPanelElement()
    End Function

End Class

Friend Class MyGroupPanelElement
    Inherits GroupPanelElement

    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(GroupPanelElement)
        End Get
    End Property

    Protected Overrides Function ArrangeOverride(finalSize As SizeF) As SizeF

        Dim clientRect As RectangleF = Me.GetClientRectangle(finalSize)

        Dim sizeGripRect As New RectangleF(clientRect.X, clientRect.Bottom - Me.SizeGrip.DesiredSize.Height, clientRect.Width, Me.SizeGrip.DesiredSize.Height)
        Me.SizeGrip.Arrange(sizeGripRect)
        clientRect.Height -= Me.SizeGrip.DesiredSize.Height

        Dim groupHeaderRect As New RectangleF(clientRect.X, clientRect.Y, Me.Header.DesiredSize.Width, clientRect.Height)
        Me.Header.Arrange(groupHeaderRect)
        clientRect.Width -= Me.Header.DesiredSize.Width

        Dim scrollViewRect As New RectangleF(clientRect.X + Me.Header.DesiredSize.Width, clientRect.Y, clientRect.Width, clientRect.Height)
        If scrollViewRect.Width > 20 Then
            Me.ScrollView.Arrange(scrollViewRect)
        End If

        Return finalSize

    End Function

End Class

How to reproduce: check the attached video

Workaround: disable pinning of the last row when it is in a group
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.ContextMenuOpening += RadGridView1_ContextMenuOpening;
    }

    private void RadGridView1_ContextMenuOpening(object sender, Telerik.WinControls.UI.ContextMenuOpeningEventArgs e)
    {
        GridRowHeaderCellElement header = e.ContextMenuProvider as GridRowHeaderCellElement;
        if (header != null && header.RowInfo.Group != null)
        {
            var notPinned = header.RowInfo.Group.GroupRow.ChildRows.Where(r => r.IsPinned == false).ToList();
            if (notPinned.Count <= 1)
            {
                e.ContextMenu.Items.RemoveAt(0);
                e.ContextMenu.Items.RemoveAt(0);
            }
        }
    }

    private DataTable GetData()
    {
        dt = new DataTable();

        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 2; i++)
        {
            for (int j = 0; j < 2; j++)
            {
                dt.Rows.Add(i, "Name " + i + " " + j, DateTime.Now.AddDays(i), i % 2 == 0);
            }
        }

        return dt;
    }
}

How to reproduce: 
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        dt.Clear();
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "New Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }
    }

    private DataTable GetData()
    {
        dt = new DataTable();

        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 500; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }

        return dt;
    }
}

Workaround:
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.RowsChanged += RadGridView1_RowsChanged;
    }

    private void RadGridView1_RowsChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
    {
        if (e.Action == Telerik.WinControls.Data.NotifyCollectionChangedAction.Reset)
        {
            this.radGridView1.MasterView.PinnedRows.GetType().GetMethod("Clear", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)
                .Invoke(this.radGridView1.MasterView.PinnedRows, new object[] { });
        }
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        dt.Clear();
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "New Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }
    }

    private DataTable GetData()
    {
        dt = new DataTable();

        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 500; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }

        return dt;
    }
}

Workaround:

AddHandler Me.RadGridView1.CreateCompositeFilterDialog, AddressOf CreateCompositeFilterDialog

Private Sub CreateCompositeFilterDialog(sender As Object, e As GridViewCreateCompositeFilterDialogEventArgs)
    Dim dialog As New CompositeDataFilterForm()
    AddHandler dialog.DataFilter.EditorRequired, AddressOf EditorRequired
    e.Dialog = dialog
End Sub

Private Sub EditorRequired(sender As Object, e As TreeNodeEditorRequiredEventArgs)
    Dim filterNode As DataFilterCriteriaNode = TryCast(e.Node, DataFilterCriteriaNode)
    If filterNode IsNot Nothing AndAlso filterNode.PropertyName = "BASE_NULL_DATE" AndAlso TypeOf sender Is DataFilterValueEditorElement Then
        e.Editor = New TreeViewDateTimeEditor()
    End If
End Sub

To reproduce:
private void RadGridView1_KeyPress(object sender, KeyPressEventArgs e)
{
    int startIndex = radGridView1.CurrentRow.Index +1;

    for (int i = startIndex; i < radGridView1.Rows.Count; i++)
    {
        var value = radGridView1.Rows[i].Cells["Name"].Value.ToString().ToLower();
        if (value.StartsWith(e.KeyChar.ToString()))
        {
            radGridView1.TableElement.ScrollToRow(radGridView1.Rows[i]);
            radGridView1.ClearSelection();
            radGridView1.Rows[i].IsSelected = true;
            radGridView1.Rows[i].IsCurrent = true;
            radGridView1.Columns[1].IsCurrent = true;
            break;
        }
    }
}

- Press some key so the grid is scrolled down, then press the down key.
- The first row is selected instead of the next one. 

Workaround:
remove this line:
radGridView1.Rows[i].IsSelected = true;

To reproduce:
1. Run the attached sample project.
2. Right-click the grid.
3. Quickly navigate to another application (e.g. Windows Explorer) before the context menu is shown & wait at windows explorer to display the menu
4. The context will be stuck.

To reproduce:
this.radGridView1.MultiSelect = true;
this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

radGridView1.ShowRowHeaderColumn = false;

Then select the cells in the firs column by dragging the mouse.

Workaround:
// radGridView1.ShowRowHeaderColumn = false;
radGridView1.TableElement.RowHeaderColumnWidth = 0;

How to reproduce:
public partial class Form1 : Form
{
    private RadGridView grid = new RadGridView();

    public Form1()
    {
        InitializeComponent();

        Controls.Add(grid);
        grid.Dock = DockStyle.Fill;
        grid.DataSource = this.GetData();
    }

    private object GetData()
    {
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("Id", typeof(int));
        dataTable.Columns.Add("Name", typeof(string));
        dataTable.Columns.Add("Checked", typeof(bool));

        for (int i = 0; i < 200; i++)
        {
            dataTable.Rows.Add(i, "Name " + i, i % 2 == 0);
        }

        return dataTable;
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewCheckBoxColumn checkBoxColumn = (GridViewCheckBoxColumn)grid.Columns["Checked"];
        checkBoxColumn.EnableHeaderCheckBox = true;
        checkBoxColumn.SortOrder = RadSortOrder.Ascending;
    }
}

Workaround: 
public partial class Form1 : Form
{
    private RadGridView grid = new RadGridView();

    public Form1()
    {
        InitializeComponent();

        Controls.Add(grid);
        grid.Dock = DockStyle.Fill;
        grid.DataSource = this.GetData();

        grid.MouseDown += grid_MouseDown;
        grid.MouseUp += grid_MouseUp;
    }

    private object GetData()
    {
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("Id", typeof(int));
        dataTable.Columns.Add("Name", typeof(string));
        dataTable.Columns.Add("Checked", typeof(bool));

        for (int i = 0; i < 200; i++)
        {
            dataTable.Rows.Add(i, "Name " + i, i % 2 == 0);
        }

        return dataTable;
    }

    private void grid_MouseDown(object sender, MouseEventArgs e)
    {
        RadGridView grid = (RadGridView)sender;
        RadCheckBoxElement cell = grid.ElementTree.GetElementAtPoint(e.Location) as RadCheckBoxElement;
        if (cell != null && cell.Parent is GridCheckBoxHeaderCellElement)
        {
            sw = new Stopwatch();
            sw.Start();
            grid.BeginUpdate();
        }
    }

    Stopwatch sw;
    private void grid_MouseUp(object sender, MouseEventArgs e)
    {
        RadGridView grid = (RadGridView)sender;
        RadCheckBoxElement cell = grid.ElementTree.GetElementAtPoint(e.Location) as RadCheckBoxElement;
        if (cell != null && cell.Parent is GridCheckBoxHeaderCellElement)
        {
            grid.EndUpdate();
            sw.Stop();
            Console.WriteLine(sw.ElapsedMilliseconds);
        }
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewCheckBoxColumn checkBoxColumn = (GridViewCheckBoxColumn)grid.Columns["Checked"];
        checkBoxColumn.EnableHeaderCheckBox = true;
        checkBoxColumn.SortOrder = RadSortOrder.Ascending;
    }
}

To reproduce: please refer to the attached sample project and follow the steps in the provided gif file: 
1. Enter something in the search bar
2. Filter on some column, uncheck "All", then select a few items, then validate
3. Click on a random row on the grid
4. Filter on the same column as in 2., check "All", then uncheck a few items, then validate

Workaround:

private void radGridView1_FilterChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
{
    this.radGridView1.MasterTemplate.Refresh();
}

Please refer to the attached sample project and gif file illustrating the behavior.
ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition;

            foreach (GridViewColumnGroup group in view.ColumnGroups)
            {
                group.ShowHeader = false;
            }

Workaround: use the ViewCellFormatting and enable the back color for the header cells that are over the hidden group headers

public RadForm1()
{
    InitializeComponent();

    ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition ;
    foreach (GridViewColumnGroup group in view.ColumnGroups)
    {
        group.ShowHeader = false;
    }
}

private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Row is GridViewTableHeaderRowInfo && !(e.CellElement is GridColumnGroupCellElement))
    {
        e.CellElement.DrawFill = true;
        e.CellElement.BackColor = Color.FromArgb(234, 242, 252);
        e.CellElement.GradientStyle = GradientStyles.Solid;
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
    }
}

How to reproduce:
public partial class Form1 : Form
{
    public class Data
    {
        public bool Checked { get; set; }
        public string Text { get; set; }
    }

    private readonly RadGridView _grid;

    private readonly List<Data> _dataList = new List<Data>
    {
        new Data {Text = "abc"},
        new Data {Text = "def"},
        new Data {Text = "ghi"},
        new Data {Text = "jkl"},
        new Data {Text = "mno"},
        new Data {Text = "pqr"},
        new Data {Text = "stu"},
        new Data {Text = "vwx"},
        new Data {Text = "yz0"}
    };

    public Form1()
    {
        InitializeComponent();

        _grid = new RadGridView
        {
            Dock = DockStyle.Fill,
            AllowSearchRow = true
        };

        Controls.Add(_grid);

        _grid.DataSource = _dataList;
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
        
        _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
        _grid.MasterView.TableSearchRow.Search("abc");
    }
}

Workaround: clear the search text
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
    
    _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
    _grid.MasterView.TableSearchRow.Search("abc");

    this._grid.MouseDown += _grid_MouseDown;
}

private void _grid_MouseDown(object sender, MouseEventArgs e)
{
    RadGridView grid = (RadGridView)sender;
    RadCheckBoxElement cell = grid.ElementTree.GetElementAtPoint(e.Location) as RadCheckBoxElement;
    if (cell != null && cell.Parent is GridCheckBoxHeaderCellElement && grid.SortDescriptors.Count > 0))
    {
        grid.MasterView.TableSearchRow.Search("");
    }
}

To reproduce:
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    DataTable dt = NewDt();
    this.radGridView1.DataSource = dt;      
    var c = this.radGridView1.Columns["Calculted"] as GridViewDecimalColumn;
    c.EnableExpressionEditor = true;
    c.Expression = "Column2 / SUM(Column2) * 10";
    c.DecimalPlaces = 2;
    c.FormatString = "{0:N2}";

    this.radGridView1.GroupDescriptors.Add("GroupByColumn", ListSortDirection.Ascending);
}

private DataTable NewDt()
{
    var dt = new DataTable();
    dt.Columns.Add("ItemColumn");
    dt.Columns.Add("GroupByColumn");
    dt.Columns.Add("Column2", typeof(decimal));
    dt.Columns.Add("Calculted", typeof(decimal));

    for (int i = 1; i <= 20; i++)
    {
        string gr = string.Empty;
        if (i % 3 == 0)
            gr = "gr3";
        else if (i % 2 == 0)
            gr = "gr2";
        else if (i % 5 == 0)
            gr = "gr5";
        else
            gr = "item" + i.ToString();

        dt.Rows.Add("id #" + i.ToString(), gr, i, 0);
    }
    return dt;
}

Workaround:
public class CustomExpressionContext : Telerik.Data.Expressions.ExpressionContext
{

    RadGridView radGridView;
    public CustomExpressionContext(RadGridView grid)
    {
        this.radGridView = grid;
    }

    public decimal MySum(string columnName)
    {
        decimal sum = 0;
        
        foreach (var item in radGridView.Rows)
        {
            sum += (decimal)item.Cells[columnName].Value;
        }
        return sum;
    }
}

 Telerik.Data.Expressions.ExpressionContext.Context = new CustomExpressionContext(this.radGridView1);