Workaround: 

private void radRichTextEditor1_CommentShowing(object sender, Telerik.WinForms.Documents.UI.CommentShowingEventArgs e)
{
    this.radRichTextEditor1.RichTextBoxElement.ContextMenu.Opened-=ContextMenu_Opened;
    this.radRichTextEditor1.RichTextBoxElement.ContextMenu.Opened+=ContextMenu_Opened;
     
}
 
private void ContextMenu_Opened(object sender, Telerik.WinForms.Documents.UI.Extensibility.ContextMenuPlacementEventArgs e)
{
    this.radRichTextEditor1.RichTextBoxElement.ContextMenu.Hide();
}

Workaround:

private void radRichTextEditor1_CommentShowing(object sender, Telerik.WinForms.Documents.UI.CommentShowingEventArgs e)
{
  ((SelectionMiniToolBar)  this.radRichTextEditor1.RichTextBoxElement.SelectionMiniToolBar).Shown+=Form1_Shown;    
}
 
private void Form1_Shown(object sender, EventArgs e)
{
    ((SelectionMiniToolBar)sender).Visible = false;
    ((SelectionMiniToolBar)sender).VisibleChanged+=Form1_VisibleChanged;
}
 
private void Form1_VisibleChanged(object sender, EventArgs e)
{
    ((SelectionMiniToolBar)sender).Visible = false; 
}

In some cases where you have RTL word and a number, the word order is different than the order displayed in MS Word (see attached doc). This might be related to the applied styles, because if you apply normal style, the order will be the same as in RadRichTextEditor.

Workaround: specify the users' dictionary with colors when the user is changed.
 
private void radRichTextEditor1_UserInfoChanged(object sender, EventArgs e)
{
    FieldInfo fi = typeof(TrackChangesOptions).GetField("userToColorMap", BindingFlags.Instance | BindingFlags.NonPublic) ;
    Dictionary<string, Telerik.WinControls.RichTextEditor.UI.Color> userToColorMap = fi.GetValue(radRichTextEditor1.RichTextBoxElement.TrackChangesOptions)
        as Dictionary<string, Telerik.WinControls.RichTextEditor.UI.Color>;
    userToColorMap = new Dictionary<string, Telerik.WinControls.RichTextEditor.UI.Color>();
    userToColorMap.Add("Boby1", Telerik.WinControls.RichTextEditor.UI.Color.FromRgb(0, 255, 255));
    userToColorMap.Add("Boby2", Telerik.WinControls.RichTextEditor.UI.Color.FromRgb(255, 0, 255));
    fi.SetValue(radRichTextEditor1.RichTextBoxElement.TrackChangesOptions, userToColorMap);
}

To reproduce:

1. Install a font that is not part of the default ones.
2. Enter some text in RadRichTextEditor and use the installed font.
3. Export to pdf.
4. Uninstall the font.
5. Try to open the exported pdf. You will notice that the font is not embedded into the file and can not render the text correctly. Please refer to the attached sample pdf file with custom font.

To reproduce:
-  Use the cut option in the context menu then  dispose the rich text editor and collect the garbage.

Workaround:
Telerik.WinControls.RichTextEditor.UI.ContextMenu menu = textEditor.RichTextBoxElement.ContextMenu as Telerik.WinControls.RichTextEditor.UI.ContextMenu;
textEditor.RichTextBoxElement.ContextMenu = null;

RadDropDownMenu ddm = menu.GetType().GetField("radDropDownMenu", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(menu) as RadDropDownMenu;

Workaround: set the FormBorderStyle property to FixedSingle for the respective dialog.

RadForm f = this.radRichTextEditor1.RichTextBoxElement.FontPropertiesDialog as RadForm;
f.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedSingle;

To reproduce: use the following code snippet and refer to the attached screenshot. Cells' width should be identical in both of the cases.

RadDocument document = new RadDocument();
Section section = new Section();
Table t = new Table();
t.Borders = new TableBorders(new Border(2, Telerik.WinForms.Documents.Model.BorderStyle.Single, Color.Black));
TableRow r1 = new TableRow();
TableCell cell = new TableCell();
Paragraph p = new Paragraph();
Span s = new Span();
s.Text = "Header";
cell.ColumnSpan = 3;
p.Inlines.Add(s);
cell.Blocks.Add(p);
cell.PreferredWidth = new TableWidthUnit(TableWidthUnitType.Percent, 100);
r1.Cells.Add(cell);

TableRow r2 = new TableRow();
TableCell cell11 = new TableCell();
Paragraph p11 = new Paragraph();
Span s11 = new Span();
s11.Text = "Cell1,1";
p11.Inlines.Add(s11);
cell11.Blocks.Add(p11);
cell11.PreferredWidth = new TableWidthUnit(TableWidthUnitType.Percent, 40);

TableCell cell12 = new TableCell();
Paragraph p12 = new Paragraph();
Span s12 = new Span();
s12.Text = "Cell1,2";
p12.Inlines.Add(s12);
cell12.Blocks.Add(p12);
cell12.PreferredWidth = new TableWidthUnit(TableWidthUnitType.Percent, 10);

TableCell cell13 = new TableCell();
Paragraph p13 = new Paragraph();
Span s13 = new Span();
s13.Text = "Cell1,3";
p13.Inlines.Add(s13);
cell13.Blocks.Add(p13);
cell13.PreferredWidth = new TableWidthUnit(TableWidthUnitType.Percent, 50);

r2.Cells.Add(cell11);
r2.Cells.Add(cell12);
r2.Cells.Add(cell13);
t.Rows.Add(r1);
t.Rows.Add(r2);
section.Blocks.Add(t);
document.Sections.Add(section);
this.radRichTextEditor1.Document = document;

Workaround: do not specify the PreferredWidth proeprty for cells with ColumnSpan>1

To reproduce:

RadDocument document = new RadDocument();
Section section = new Section();
Paragraph p = new Paragraph();
Span span = new Span();
span.Text ="test" + Environment.NewLine + "test";
ReadOnlyRangeStart rangeStart = new ReadOnlyRangeStart();
ReadOnlyRangeEnd rangeEnd = new ReadOnlyRangeEnd();
rangeEnd.PairWithStart(rangeStart);
p.Inlines.Add(rangeStart);
p.Inlines.Add(span);
p.Inlines.Add(rangeEnd);
section.Blocks.Add(p);
document.Sections.Add(section);
this.radRichTextEditor1.Document = document;


Workaround: add an empty span as last inline element in the paragraph:

RadDocument document = new RadDocument();
Section section = new Section();
Paragraph p = new Paragraph();
Span span = new Span();
span.Text ="test" + Environment.NewLine + "test";
ReadOnlyRangeStart rangeStart = new ReadOnlyRangeStart();
ReadOnlyRangeEnd rangeEnd = new ReadOnlyRangeEnd();
rangeEnd.PairWithStart(rangeStart);
p.Inlines.Add(rangeStart);
p.Inlines.Add(span);
p.Inlines.Add(rangeEnd);
///////////////////////////////////////
Span emptySpan = new Span();
emptySpan.Text = " ";
p.Inlines.Add(emptySpan);
///////////////////////////////////////
section.Blocks.Add(p);
document.Sections.Add(section);
this.radRichTextEditor1.Document = document;

To reproduce:
- Set the ParagraphDefaultSpacingAfter to 1.
- Start the application and add a comment directly.

The issue exist when there is many comments as well. The comment balloons should have minimum size and should not overlap eachother.

Workaorund:
void radRichTextEditor1_CommandExecuting(object sender, CommandExecutingEventArgs e)
{
    if (e.Command is InsertCommentCommand)
    {
        int paragaphs = radRichTextEditor1.Document.Sections.First().Blocks.Count();

        if (paragaphs <= 1)
        {
            radRichTextEditor1.InsertLineBreak();
        }

    }
}
 

When some commands are executed the RibbonUI is not updated.
For example:
private void radButton1_Click(object sender, EventArgs e)
{
    ToggleCommentsCommand command = new ToggleCommentsCommand(radRichTextEditor1.RichTextBoxElement);
    command.Execute();
}

Workaround:
var button = ribbonBar1.GetType().GetField("toggleButtonShowHideComments", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(this.ribbonBar1) as RadToggleButtonElement;
button.ToggleState = (radRichTextEditor1.RichTextBoxElement.ShowComments == true) ? ToggleState.On : ToggleState.Off;          

To reproduce:
- Add some comments and try to navigate through them.
- Try add or show/hide the comments while the cursor is in one of the comments.

Workaround:
Create a custom RichTextEditorRibbonBar
and override the following methods:

Public Class MyRichTextEditorRibbonBar
    Inherits RichTextEditorRibbonBar

    Protected Overrides Sub ButtonNewComment_Click(sender As Object, e As EventArgs)
        Dim command As New InsertCommentCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)

    End Sub

    Protected Overrides Sub ButtonDeleteComment_Click(sender As Object, e As EventArgs)
        Dim command As New DeleteCommentCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)
    End Sub

    Protected Overrides Sub ButtonPreviousComment_Click(sender As Object, e As EventArgs)
        Dim command As New GoToPreviousCommentCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)
    End Sub

    Protected Overrides Sub ButtonNextComment_Click(sender As Object, e As EventArgs)
        Dim command As New GoToNextCommentCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)
    End Sub

    Protected Overrides Sub ToggleButtonShowHideComments_ToggleStateChanged(sender As Object, args As StateChangedEventArgs)

        Dim command As New ToggleCommentsCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)
    End Sub

    Protected Overrides Sub ButtonDeleteAllComments_Click(sender As Object, e As EventArgs)
        Dim command As New DeleteAllCommentsCommand(Me.AssociatedRichTextEditor.RichTextBoxElement)
        Me.ExecuteCommand(command)
    End Sub

    Protected Overrides Sub HandleDocumentCommandExecuted(document As RadDocument)
        MyBase.HandleDocumentCommandExecuted(document)

        Dim hasComments As Boolean = Me.AssociatedRichTextEditor.Document.EnumerateChildrenOfType(Of CommentRangeStart)().Count() > 0

        Me.buttonPreviousComment.Enabled = hasComments
        Me.buttonNextComment.Enabled = hasComments
        Me.buttonDeleteComment.Enabled = hasComments
        Me.buttonDeleteAllComments.Enabled = hasComments
    End Sub

End Class

To reproduce:
1.Use the provided xml file in the following article: http://www.telerik.com/help/winforms/richtexteditor-localization.html and change these strings:

"Documents_ParagraphPropertiesDialog_TextAlignment_Center": "Mittig";
"Documents_ParagraphPropertiesDialog_TextAlignment_Justify": "Block";
"Documents_ParagraphPropertiesDialog_TextAlignment_Left": "Links";
"Documents_ParagraphPropertiesDialog_TextAlignment_Right": "Rechts";

2.Afterwards,  specify the RichTextBoxLocalizationProvider.CurrentProvider by loading the modified file. 
3.Run the application, select some paragraph and try to change its alignment. As a result when the ParagraphPropertiesDialog is opened, incorrect alignment text is displayed in the drop-down. If you select a new value a handled ArgumentException is thrown and the paragraph is not aligned at all.

Workaround: use a custom ParagraphPropertiesDialog:
public Form1()
{
    RichTextBoxLocalizationProvider.CurrentProvider = RichTextBoxLocalizationProvider.FromFile(@"..\..\RichTextBoxStrings - Copy.xml");
     
    InitializeComponent();

    this.radRichTextEditor1.RichTextBoxElement.ParagraphPropertiesDialog = new CustomDialog();
}


public partial class CustomDialog : ParagraphPropertiesDialog
{
    public CustomDialog()
    {
        InitializeComponent();
       
        FieldInfo fi = typeof(ParagraphPropertiesDialog).GetField("radButtonOK", 
            BindingFlags.Instance | BindingFlags.NonPublic);
        RadButton btn = fi.GetValue(this) as RadButton;
     
        btn.MouseDown += btn_MouseDown;
    }
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e); 
        if (selectedItem != null)
        {
            ButtonAlignement.Text = selectedItem.Text;
        }
    }

    public RadDropDownButton ButtonAlignement
    {
        get
        { 
            FieldInfo fi = typeof(ParagraphPropertiesDialog).GetField("radDropDownButtonAlignement",
                BindingFlags.Instance | BindingFlags.NonPublic);
            RadDropDownButton radDropDownButtonAlignement = fi.GetValue(this) as RadDropDownButton;
            return radDropDownButtonAlignement;
        }
    }

    RadMenuItem selectedItem = null;
    RadDropDownButton radDropDownButtonAlignement = null;

    private void btn_MouseDown(object sender, MouseEventArgs e)
    {
        foreach (RadMenuItem item in ButtonAlignement.Items)
        {
            if (item.Text == ButtonAlignement.Text)
            {
                selectedItem = item;
                break;
            }
        }
        if (selectedItem != null)
        {
            ButtonAlignement.Text = selectedItem.Tag.ToString();
        }
    }
}

To reproduce: use the code snippet below. Please refer to the attached screenshot.

public Form1()
{
    RichTextBoxLocalizationProvider.CurrentProvider = new MyRichTextBoxLocalizationProvider();
    InitializeComponent();
}

public class MyRichTextBoxLocalizationProvider : RichTextBoxLocalizationProvider
{
    public override string GetLocalizedString(string id)
    { 

        return base.GetLocalizedString(id);
    }
}

Workaround: 
Modify the XML file containing all the strings RichTextBoxLocalizationProvider.CurrentProvider = RichTextBoxLocalizationProvider.FromFile(@"C:\RichTextBoxStrings.xml");. You can download a XML file that contains all the currently used strings from here: http://www.telerik.com/docs/default-source/ui-for-winforms/richtextboxstrings.zip?sfvrsn=2

Currently, the scrollbars width is calculated only according to  SystemInformation.VerticalScrollBarWidth.

Workaround:
class MyRTE : RadRichTextEditor
{
    protected override void CreateChildItems(RadElement parent)
    {
        var rte = new MyRTEElement();
        typeof(RadRichTextEditor).GetField("richTextBox", BindingFlags.NonPublic | BindingFlags.Instance).SetValue(this, rte);
        parent.Children.Add(rte);

    }
}
public class MyRTEElement : RadRichTextBox
{
    protected override SizeF ArrangeOverride(SizeF finalSize)
    {
        base.ArrangeOverride(finalSize);


        System.Drawing.RectangleF finalRect = new System.Drawing.RectangleF(System.Drawing.PointF.Empty, finalSize);
        System.Drawing.RectangleF clientRect = finalRect;

        System.Drawing.RectangleF horizontalScrollRect = System.Drawing.RectangleF.Empty;
        System.Drawing.RectangleF verticalScrollRect = System.Drawing.RectangleF.Empty;
        System.Drawing.RectangleF presenterArea = clientRect;

        if (this.VerticalScrollBar.Visibility != Telerik.WinControls.ElementVisibility.Collapsed)
        {
            verticalScrollRect = new System.Drawing.RectangleF(
                new System.Drawing.PointF(finalRect.Right - this.VerticalScrollBar.Size.Width, finalRect.Top),
                new System.Drawing.SizeF(this.VerticalScrollBar.Size.Width, finalRect.Height));
        }

        if (this.HorizontalScrollBar.Visibility != Telerik.WinControls.ElementVisibility.Collapsed)
        {
            horizontalScrollRect = new System.Drawing.RectangleF(
                new System.Drawing.PointF(finalRect.Left, finalRect.Bottom - this.HorizontalScrollBar.Size.Height),
                new System.Drawing.SizeF(finalRect.Width, this.HorizontalScrollBar.Size.Height));
            if (this.VerticalScrollBar.Visibility != Telerik.WinControls.ElementVisibility.Collapsed)
            {
                horizontalScrollRect.Width -= this.VerticalScrollBar.Size.Width;
                verticalScrollRect.Height -= this.HorizontalScrollBar.Size.Height;
            }
        }

        presenterArea.Width -= verticalScrollRect.Width;
        presenterArea.Height -= horizontalScrollRect.Height;

        if (this.RightToLeft)
        {
            horizontalScrollRect = Telerik.WinControls.Layouts.LayoutUtils.RTLTranslateNonRelative(horizontalScrollRect, clientRect);
            verticalScrollRect = Telerik.WinControls.Layouts.LayoutUtils.RTLTranslateNonRelative(verticalScrollRect, clientRect);
            presenterArea = Telerik.WinControls.Layouts.LayoutUtils.RTLTranslateNonRelative(presenterArea, clientRect);
        }

        this.VerticalScrollBar.Arrange(verticalScrollRect);
        this.HorizontalScrollBar.Arrange(horizontalScrollRect);
        var contentPresenter = typeof(RadRichTextBox).GetField("contentPresenter", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).GetValue(this) as ContentControl;
        contentPresenter.Arrange(presenterArea);

        return finalSize;
    }
    protected override SizeF MeasureOverride(SizeF availableSize)
    {
        base.MeasureOverride(availableSize);

        System.Drawing.SizeF horizontalScrollSize = System.Drawing.SizeF.Empty;
        System.Drawing.SizeF verticalScrollSize = System.Drawing.SizeF.Empty;
        System.Drawing.SizeF presenterSize = availableSize;

        if (this.VerticalScrollBar.Visibility != Telerik.WinControls.ElementVisibility.Collapsed)
        {
            verticalScrollSize = new System.Drawing.SizeF(this.VerticalScrollBar.Size.Width, availableSize.Height);
        }

        if (this.HorizontalScrollBar.Visibility != Telerik.WinControls.ElementVisibility.Collapsed)
        {
            horizontalScrollSize = new System.Drawing.SizeF(availableSize.Width, this.HorizontalScrollBar.Size.Height);
        }

        presenterSize.Width -= verticalScrollSize.Width;
        presenterSize.Height -= horizontalScrollSize.Height;
        //presenterSize = base.GetClientRectangle(presenterSize).Size;

        this.VerticalScrollBar.Measure(verticalScrollSize);
        this.HorizontalScrollBar.Measure(horizontalScrollSize);

        var contentPresenter = typeof(RadRichTextBox).GetField("contentPresenter", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).GetValue(this) as ContentControl;
        contentPresenter.Measure(presenterSize);

        return availableSize;
    }
    
}

To reproduce:
<!DOCTYPE html>
<html>
    <head>
        <style type="text/css">body {
            font-family: Segoe UI;
        } a {
            color: #5cb85c;
        } a:hover {
            color: #000000;
        }</style>
        <title>My first HTML document</title>
    </head>
    <body>
        <span style="font-size: 14px; margin-bottom: 5px; display:block;">The Title</span>
        <ul style="list-style-type:none; padding-left: 0px; margin-left: 0px;">
            <li style="margin-left: 0px; padding-left: 0px; font-size: 12px;">
                <a href="www.myurl.com">My URL</a>
            </li>
            <li style="margin-left: 0px; padding-left: 0px; font-size: 12px;">
                <a href="https://www.myurl2.com/">My URL 2</a>
            </li>
        </ul>
    </body>
</html>

To reproduce:

public Form1()
{
    InitializeComponent();
    this.radRichTextEditor1.IsReadOnly = true;
}

Workaround:
private void Form1_Load(object sender, EventArgs e)
{
    this.radRichTextEditor1.IsReadOnly = true;
    Telerik.WinControls.RichTextEditor.UI.DocumentWebLayoutPresenter webLayoutPresenter = 
        this.radRichTextEditor1.RichTextBoxElement.ActiveEditorPresenter as Telerik.WinControls.RichTextEditor.UI.DocumentWebLayoutPresenter;
    webLayoutPresenter.Caret.Width = 0;
}