I have just played a youtube video (coincidentally the youtube message from
AMD's John Byrne regarding Catalyst!) and I have just checked that the
following messages show up in the dmesg log:

[ 2053.298531] radeon 0000:05:00.0: Packet0 not allowed!
[ 2058.548486] radeon 0000:05:00.0: Packet0 not allowed!
[ 2166.793537] radeon 0000:05:00.0: Packet0 not allowed!

Given that the video is very short and I haven't played any other youtube video
the system hasn't crashed (yet). I presume that if I were to play other videos
the system would hang.