Sometimes (very rarely actually) I have a problem that when I try to shut down my desktop, it will freeze with the following screen and I can only shut it down by pressing the power button for 8 seconds:

Does anyone know why this error occurs? I am guessing it is because of the proprietary fglrx-driver. How I could get more diagnostic information if it reoccurs? I guess there would be a lot more information than I can capture with a simply photo, but as soon as the screen appears it is impossible to do anything.
The problem is, it occurs very rarely (maybe once a month) so it is not really a big hassle and I can not willingly recreate it. I would just like to know what exactly is causing it.
Edit: After stumbling over this bug - this seems to be the reason for it: the fglrx driver seems to be buggy.