I haven't pinpointed any potential triggers yet, but sometimes when I come out of sleep or from having a locked screen, after typing my password in and re-entering the OS, about 20 or 30 seconds later, the screen just shuts off and goes to sleep.
I almost always use an external monitor, but can't remember if this ever happens w/out it connected.
One thing that does happen every time is that the password box is missing its blue highlight. My keyboard focus is still inside of it though. When I see this, 100% of the time, this re-lock/sleep happens.
I'm running 10.8.2 mountain lion.
Any ideas on what is causing this, or anyone else experience this?