Bookmarks are not imported and missing from a specific document scenario.

Workaround: Import and then export the document using the WordsProcessing library and then open it with the editor.

The GetSupportedExtensions method of DocumentFormatProvidersManager throws NullReferenceException when invoked.

Workaround:

HashSet<string> supportedExtensions = new HashSet<string>();
foreach (var provider in DocumentFormatProvidersManager.FormatProviders)
{
    foreach (var extension in provider.SupportedExtensions)
    {
        supportedExtensions.Add(extension);
    }
}

In certain rare occasions, double-clicking a table cell may throw:

inline does not belong to this document.
at Telerik.Windows.Documents.Model.RadDocument.GetContainingAnnotationRanges[TRangeStart](Inline inline, Predicate`1 filter, Boolean inclusive)
at Telerik.Windows.Documents.Selection.MouseSelectionHandler.HandleCheckBox(InlineLayoutBox layoutBox)
at Telerik.Windows.Documents.Selection.MouseSelectionHandler.RegisterDocumentSingleMouseDown(Boolean ctrlPressed, Boolean shiftPressed, Point position, UIElement originalSource)

. . .

Type a few lines in RadRichTextBox.
Double-clicking within a word, holding down the mouse button on the second click, and then drag it. 

Actual: Dragging will move the word instead of extending the selection as it should.
Expected: The selection should be extended. Moving the word shouldn't happen unless you release after the double click and then click and drag. 

After deleting a custom annotation using the DeleteAnnotationRange method when there is a text immediately in front of the annotation start:

and then trying to delete the last character in this former annotation range an exception is thrown (Exception: System.InvalidOperationException: 'inline does not belong to this document.')