Suddenly my screen looked like this:
The pixelated areas are made with GIMP afterwards for privacy reasons. The green lines are the thing I am wondering about. After a reboot everything was fine again. I really don't have any clue how this could happen. Any ideas?
Unfortunately I can't provide any further information. It did not happen again and logs are lost now. The problem occured on a LTSP client using 12.04.2 with XFCE.