Comment 4 for bug 987498

It may be related to Thunderbird when it received a new mail, but only sometime. Indeed I'm able to get a failsafe desktop when unity/compiz freeze by using unity-2d/metacity, and then I notice that Thunderbird window that was minimized cannot display it content (unity-2d show a dark window). I've noticed that twice so far, so I'm still not sure but it may help.

To get back on unity-2d/metacity do the following things:
Ctrl-Alt-F1
[wait until it switch; that can take about 30 seconds]
[login]
killall -9 compiz
DISPLAY=0:0 metacity --replace &
DISPLAY=0:0 unity-2d-panel &
DISPLAY=0:0 unity-2d-shell &
killall -9 compiz
Ctrl-Alt-F7