I just had that about 30 minutes ago, only happened one time. On a Late 2013 15" rMBP. It happened right after quitting Diablo 3 so I assumed it had something to with that. I could still see everything perfectly, and pressing the power button quickly still logged me out, but I could not type my password. Had to power it off. Submitted the bug to Apple.
Is happening to me every two or three hours. Driving me crazy!