so i was just using firefox and i was on facebook and all of a sudden my iMac's screen had a scroll of a dark shade of gray run down the screen. A message popped up saying that i needed to forcefully shut my computer down. So i turned it off with force! (holding the power button). I turned it on and this message popped up when i logged in. (look at attachment). Soon after, about 3 minutes later, the same thing happened again (my computer "crashed") and I was using Opera this time. I was just wondering if other users have ever experienced this before. Please comment.