I have noticed a strange behavior with the autocomplete box. I have an autocompletebox bound to an asp sql datasource and I have set an empty message to it. If I copy some text and paste into the input, the text gets added to the empty message.
This was a bug present in the earlier versions of RadControls and this has been fixed in the Q2 2013 Beta (2013.2.522) version of RadControls for ASP.NET AJAX. So you can update to this version or the Q3 release to get this bug fixed.