Add support for shapes, and especially for shapes with textual content. 
In OOXML, shapes are represented by the wps:wsp element, and shapes with textual content by <wps:txbx>, <w:txbxContent>.
Such shape can be added to a Word document using the Insert -> Text -> Text Box -> Draw Text Box, or through a shape's context menu -> Add Text.

Note: Do not confuse with Text/Rich Text content controls (https://feedback.telerik.com/Project/143/Feedback/Details/113730 ).

When the Track Changes is On and we try to change the style it applies the first time but the second try throws an exception and the style doesn't apply: System.ArgumentNullException: 'Value cannot be null. Parameter name: owner'.

1. Add new text "TEST".
2. Select the text and set a new style "Heading 2", then "Heading 1".
Observed: Heading 1 doesn`t appear.

When the Track Changes is activated, a Hyperlink is inserted to a text and we try to reject changes the rejection is not applied correctly.

The result:

When the unit type is set to an inch:

Expected: to could be able to place the RulerThumb on every 1/16 of an inch.
Actual: be able to place the RulerThumb to every 1/8 of an inch. 

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);
    }
}

When "rFonts" is set in docDefaults:

<w:styles>
	<w:docDefaults>
		<w:rPrDefault>
			<w:rPr>
				<w:rFonts w:ascii="Verdana" w:eastAsia="Times New Roman" w:hAnsi="Verdana" w:cs="Verdana"/>
				...
			</w:rPr>
		</w:rPrDefault>

and partially in the local run properties:

<w:r>
	<w:rPr>
		<w:rFonts w:cs="Times New Roman"/>
		...
	</w:rPr>
	<w:t>Text</w:t>
</w:r>

A wrong font is applied.

Expected: Verdana
Actual: Times New Roman

When inserting a content control (structured document tag) into an empty RadDocument using one of the overloads of the InsertStructuredDocumentTag method accessible from RadRichTextBox and RadDocumentEditor, a NullReferenceException is thrown.

Workaround: Invoke the EnsureDocumentMeasuredAndArranged method before inserting the content control:

document.EnsureDocumentMeasuredAndArranged();

RadDocumentEditor editor = new RadDocumentEditor(document);
editor.InsertStructuredDocumentTag(SdtType.CheckBox);

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.

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)

. . .