Working with PasteOptionsPopup may cause an exception in LayoutElement.SetParent. Very difficult to reproduce, having no consistent steps. Maybe it is related to undo of paste command, but there are no other clues aside from this.

Numbered lists have invalid values when combining documents by using the InsertFragment method.

Workaround: use RadDocumentMerger:

RadDocument radDocument1 = RtfSourceProvider.Import(rtf1);
RadDocument radDocument2 = RtfSourceProvider.Import(rtf2);

AppendDocumentOptions options = new AppendDocumentOptions();
options.FirstSourceSectionPropertiesResolutionMode = SectionPropertiesResolutionMode.NoSectionBreak;

RadDocumentMerger merger = new RadDocumentMerger(radDocument1);
merger.AppendDocument(radDocument2, options);

RichText.Document = merger.Document;

Multi-byte Unicode characters can be represented as multiple ANSI characters in the form \'xx, for example:
\'89\'ce can represent the character 火 if multi-byte encoding should be used. Using multi-byte encoding could be set explicitly using the \ucN tag, or implicitly using the \fcharset for the current font (e.g. \fcharset134 is double-byte).

When such text is imported (commonly CJK text [Chinese, Korean, Japanese] the characters are imported incorrectly (as two '?' or as two different characters for each character)

Such RTF documents are produced from Office 2010 (Word, Outlook), WordPad - on saving or on copying content from the applications.

I have no idea how to reproduce this, but every so often after a paste operation into a RadRichTextBox, the host thread becomes unresponsive to user input. The message pump is still running but the following unhandled exception gets thrown in a tight infinite loop. The only way to close the window is to right click on the taskbar button.

System.NullReferenceException
Message: Object reference not set to an instance of an object.
Call Stack:
    at Telerik.Windows.Documents.Layout.LayoutElement.SetParent(LayoutElement newParent)
    at Telerik.Windows.Documents.Layout.LayoutBox.EnsureParent()
    at Telerik.Windows.Documents.Layout.LayoutBox.get_Parent()
    at Telerik.Windows.Documents.Layout.FormattingSymbolLayoutBox.get_IsEndOfCell()
    at Telerik.Windows.Documents.Layout.FormattingSymbolLayoutBox.GetFormattingSymbolVisual()
    at Telerik.Windows.Documents.SpanBoxPositionHandler.get_Location()
    at Telerik.Windows.Documents.UI.DocumentWebLayoutPresenter.GetViewPointFromDocumentPosition(DocumentPosition position)
    at Telerik.Windows.Controls.RichTextBoxUI.PasteOptionsPopup.CalculateLocation()
    at Telerik.Windows.Controls.RichTextBoxUI.PasteOptionsPopup.OnOwnerLayoutUpdated(Object sender, EventArgs e)
    at System.Windows.ContextLayoutManager.fireLayoutUpdateEvent()
    at System.Windows.ContextLayoutManager.UpdateLayout()
    at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
    at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
    at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
    at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
    at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
    at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)