Since I upgraded to Mountain Lion it happens to me every day when I enter the screen saver while having lunch it tries to restart the computer.
I don't have any clue why this happens. If there is some process running and OSX can't automatically quit it, OSX will tell me that it couldn't restart. But if there's no process like this running it will restart.
I don't really want this behaviour and searched in system settings as well in the Console in order to see why this might happen but I don't have a clue as of why.
OSX Mountain Lion 10.8.2 - MacBook Pro 15" Late 2011