The exact steps to reproduce are: 1. Use RadEditor in http://localdemos.telerik.com/aspnet-ajax-dev/editor/examples/overview/defaultcs.aspx 2. Add this HTML: <p>start</p> <p> </p> <p>end</p> 3. Go to Design; 4. Select from the empty paragraph (<p> </p>) to the end of text (<p>end</p>); 5. Copy via Ctrl+C; 6. Try to paste at the end of the "end" paragraph.
A possible solution is to set the StripFormattingOptions property to NoneSupressCleanMessage. You can test it at https://demos.telerik.com/aspnet-ajax/editor/examples/cleaningwordformatting/defaultcs.aspx.