Noticed this while while in IT Project Management class (haha).

Went on break, went to lock computer (out of habit using Windows Key + L) without thinking that the game was full screen. Upon unlock, game plays, character moves, but no actions can be performed whether it be activating a skill or selecting a menu item - including selecting the Exit Diablo III button from the menu. Had to use Alt + F4 to exit.

Details:
- North American account

System Specifics:
Thinkpad W520
Core i7 2720qm
16gb DDR3 SODIMM
Quadro 2000m 1gb video

Not game breaking per-se but certainly something of note!