On a 7950, I keep getting this error from time to time in dmesg:
radeon 0000:01:00.0: Packet0 not allowed!

I have associated this error with playing either html5 or flash videos. It may
happen when playing offline movies, but I can't tell since I haven't tested it.

When the error happens, there is a slight "stuttering" (from a fraction of
second to a few seconds). And then it continues.

There is nothing in Xorg.0.log about it, and no other message than "radeon
0000:01:00.0: Packet0 not allowed!" in dmesg.