My iMac just crashed hard. Watching a video in VLC one second, dead machine with mostly black screen the next. Some strange light colored vertical lines.
Googling around this is apparently a known problem, a hardware failure of the GPU. This Apple support page notes a particular known problem for exactly my model of video card and CPU. (2011 iMac, AMD Radeon HD 6970M GPU, 3.4GHz i7 CPU). Sounds like they have a repair program.