Comment 6 for bug 969039

Martin, all, I cannot reproduce this bug. At least, not unless I press Left slightly before Alt. But that's not what you reported.

It's possible that compiz might miss the Alt+Left keypress event if it triggers a synchronous grab in another application. Synchronous means that the application briefly takes control of the X server's key event queue and could stop them from reaching the windows below, including root which is where compiz listens for the key events. This is unavoidable and unfixable, but also just a theory. I don't yet know of any application that handles Alt+Left in that way, so further details about other apps running would be helpful.