I got that once when I went to send a PDF file to one of my friends - and found out iChat just displays it in the chat window (that's pretty cool, BTW). After that was on the screen, I started getting glitches like that every now and then.
Closing the chat window fixed it - the new chat window didn't have any problems.
Err...not until my friend sent me a small movie, which also showed up right in the chat window, anyway.