I rebooted this morning (was rebooting into Boot Camp Windows 7), and was greeted with this. I had a minor panic attack. The loading bar went across the screen, and then the system booted into Safe Mode OS X 10.8.3. I rebooted, no issue. Then I rebooted into Windows 7 - no issue. Then back into OS X. No issue.. Weird... This usually only happens on a firmware update etc, but there wasn't one. *cries* (2012 iMac, 27", fyi).