The exception is thrown by the SystemFontsManager internal class from Telerik.Windows.Documents.Core, so it can observed when using RadPdfViewer, RadRichTextBox, RadSpreadsheet.

The reason is not clear, posssible causes are:
- Corrupted fonts after upgrade from Windows XP
- Enabling of Block untrusted fonts feature (https://docs.microsoft.com/en-us/windows/threat-protection/block-untrusted-fonts-in-enterprise )

The call stack is as follows:

System.TypeInitializationException: The type initializer for 'Telerik.Windows.Documents.Core.Fonts.SystemFontsManager' threw an exception. ---> System.IO.FileNotFoundException: Unable to find the specified file.
   at MS.Internal.Text.TextInterface.Native.Util.ConvertHresultToException(Int32 hr)
   at MS.Internal.Text.TextInterface.FontList.get_Item(UInt32 A_0)
   at MS.Internal.Text.TextInterface.FontList.FontsEnumerator.get_Current()
   at MS.Internal.FontFace.TypefaceCollection.Enumerator.get_Current()
   at System.Windows.Media.Fonts.TypefaceCollection.<GetEnumerator>d__11.MoveNext()
   at Telerik.Windows.Documents.Core.Fonts.SystemFontsManager..cctor()

Workaround: 

Manually delete all registry key values in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts pointing to files outside of the Fonts folder.

NullReferenceException is thrown when some of the parts in the template of SelectionMiniToolBar are missing, e.g. bold button, italic button, etc.; and selected text properties are changed while the selection mini tool bar is still opened, for example by clicking buttons in the ribbon UI.

Available in LIB Version 2017.3.1225.

Show a preview of how a feature affects the document as the user hovers over the different options in the user interface, e.g.:
- when a style is hovered in the styles gallery, temporarily apply the style to the affected text; 
- when a font size is hovered in the font size combo (in the ribbon or in the SelectionMiniToolBar), temporarily apply the hovered font size to the affected text.

SelectionMiniToolBar and ImageMiniToolBar stay visible when keys associated with commands bindings are pressed, for example:
- When Delete key is pressed
- When arrow keys are pressed

The expected behavior is mini tool bars to hide on each key press.

Fix available in LIB Version 2017.3.1211.

Currently, blockquote tags are imported as paragraphs. Change the mechanism so they can be imported as citations.

When the tag \sl (line spacing) has value 0, according to the specification it should be calculated according to the highest character in the line, which corresponds to the LineSpacingType single. Instead, in RTB this text disappears.

Workaround: 
private void RadRichTextBox_DocumentChanged(object sender, EventArgs e)
{
    foreach (var paragraph in this.radRichTextBox.Document.EnumerateChildrenOfType<Paragraph>())
    {
        if (paragraph.LineSpacing == 0)
        {
            paragraph.LineSpacing = 1;
        }
    }
}

Fix available in LIB Version 2017.3.1211.

In Word and WordPad when hitting Ctrl+Down Arrow the cursor moves to the start of the next paragraph, until the last paragraph in the document where it moves to the end of the document. In Telerik editor it moves to the end of the paragraph.

Steps to reproduce:
- Load a document with multiple lines of text
- Click near the top of the document and hit Ctrl+Down repeatedly

Expected: The cursor moves down one paragraph at a time with the cursor at the start of the paragraph
Actual: The cursor moves down one paragraph at a time with the cursor at the end of the paragraph

Fix available in LIB Version 2017.3.1218.

When a document with NumPages field in header or footer is exported to PDF, NumPages field is evaluated wrongly - 1 for the first page, 2 for the second page, etc.

The bug is regression, introduced with R3 2017 SP1 (2017_3_1018).

Fix available in LIB Version 2017.3.1211.

Permission's range bracket disappears when it is positioned on the last line of the page.

Focus events (GotFocus, LostFocus) are raised inconsistently for RadRichTextBox, due to the internal translation of the focus to the Caret.

Scenarios:

1. Clicking inside the control, when the focus is already inside the control, triggers LostFocus, GotFocus, LostFocus, GotFocus events.

2. Switching internal editors (headers/footers) raises GotFocus and LostFocus.

3. If the control is inherited and OnGotFocus/OnLostFocus are overriden, they are never called (despite OnGotFocus one time in the beginning

The behavior is problematic, as this events are useful for custom validation scenarios.

Hyphenated words (like "good-looking", "dry-clean", "twenty-seven") should be passed to the spellchecker as a whole word. The current mechanism works well enough for English, however, for some languages and dictionaries it is necessary to pass the whole hyphenated word to the spellchecker.

When the first table row of each table (main as well as nested) has RepeatOnEveryPage set, the table header of the nested table overlaps the row on the new page.

Applying a list does not follow the specified indention of texts. When a paragraph has previously applied indentation, it should be preserved and added to the indentation of the bullet. In RadRichTextBox, the indentation of the paragraphs is not respected when applying a list style to it - only the setting of the list is applied.

Workaround is to use the Classes StylesExportMode.

Importing a document containing non-printable characters and other invalid for XML characters is possible. However, trying to export the same to XAML or DOCX causes System.ArgumentException raised by the XmlWriter.

Workaround: Preprocess the input file and remove any characters not allowed in XML.

Fix available in LIB Version 2018.3.1029.

When RadRichTextBox.DocumentInheritsDefaultStyleSettings is set to true, lists in the document are exported to HTML with Verdana font, instead of with the font applied in the UI.

Workaround: Use modification of the document default styles ("Normal") instead of setting DocumentInheritsDefaultStyleSettings  to true, e.g.:
this.radRichTextBox.Document.StyleRepository["Normal"].SpanProperties.FontFamily = new FontFamily("Segoe UI");

When deleting all content (e.g. Select All [CTRL+A] + Delete [Del]) in the document, it would be convenient for the user to preserve all formatting (local span formatting + local paragraph formatting + style) applied on the last paragraph. 

This will ensure that the user will continue edit the document with last current font, font family, etc.

If this operation is applied on an empty document, though, the styling of the single paragraph should be reset to the defaults for the document.

When a table is selected, the SelectionMiniToolbar should have a borders drop down menu which allows setting and editing of borders without entering the table borders dialog.

Case 1:
Deleting selected part from the beginning of a comment range causes the caret to appear on the next document position than the expected one.
Steps to reproduce:
- Load a document with annotation ranges
- Select from the start of the annotation ranges into the first word in the range
- Hit delete/backspace or call Delete() on the API
- The cursor will end up one position too far to the right after the content is removed

Case 2:
Annotation Ranges where SkipPositionBefore is set to false on the AnnotationRangeEnd cause caret position to jump when deleting content to the right of the Annotation Range.
Steps to reproduce:
- Open a document with Annotation Ranges where SkipPositionBefore is set to false on the AnnotationRangeEnd (the Document Protection sample in the demo app can be used)
- Move the caret to the end of the range.
- Hit the delete key
Expected: The caret remains at the end of the AnnotationRange and content to the right is deleted.
Actual: Content to the right is deleted but eventually the caret will jump to the right by one position and end up outside the Annotation Range.

List items with <li style="list-style-type:disc"> are not imported from HTML. The style works fine when it is added in the parent <ul> element. This behavior also breaks the copy/paste functionality when user copies bullet lists from a Google Document.