This is not happening all the time, but only sometimes, totally random.
When I'm in the middle of some work something totally weird happens. Keyboard suddenly stops working. I can't type, no matter which key I press nothing happens. I can't press Alt + Tab, I can't press Alt + F4, Alt + F2... nothing. Activities button is dead.
Also right click does not work, and something even more annoying, I can click on the 'menu' in the top right corner(the menu pops up), but clicking on 'power-off' or 'suspend' option doesn't do anything.
The only little thing I can do is:
Close the lid of my laptop so it goes to sleep. Than wake it up again. And than it's ok. Everything works. So I guess restarting (or refreshing) gnome-shell fixes the problem.
My specs: Ubuntu 12.04.1 LTS, Gnome3, Dell Studio laptop