Just happened a few minutes ago. I got this rMBP a week ago, and after logging in, it went back to the startup Apple logo (it just restarted on its own). After the Apple logo appeared, it automatically went back to the "password login field" -- in absence of better words -- and after logging in the second time, there was no message or crashlog. Nothing. It booted up fine and there seems to be nothing wrong as I'm typing this. Any idea on cause? Should I be worried?