If RadRichTextBox contains a table and is placed in a window and then the window is resized horizontally, the table border are moved unexpectedly and change their place relative to the content of the documen. If the window is maximized and/or restored, some of the borders disappear.

Workaround: Execute the following code snippet in window's or RadRichTextBox' SizeChanged event handler:
if (this.MyRadRichTextBox.ActiveEditorPresenter != null)
{
    this.MyRadRichTextBox.ActiveEditorPresenter.RecreateUI();
}