I have now had this happen twice, and it seems to be a mountain lion flaw. I am either logged in and switch users; or I am logged in, it goes to sleep, I wake it, and then switch users. I cannot isolate it completely, but I get a white screen. I am able to move my mouse, but everything is white. I eventually have to just shut it down. I have reset my PRAM, and at a later time even unplugged my computer for a little and rebooted. It takes a few days to get it to reoccur. I think it is isolated to the following scenario, but not positive: I am working on my user, leave my computer, come back and it is a sleep, wake it from sleep, and then switch user, and then WHITE SCREEN where mouse cursor can be moved. I am using a Mid 2010 imac.