Metacity sometimes stops recognizing keyboard shortcuts. Reproducable: 1) Choose "run program" in the menu 2) Enter "glade-2", hit enter 3) Exit it via keyboard 4) Notice that your key shortcuts no longer work. metacity-2.3.987.92-2
http://bugzilla.gnome.org/show_bug.cgi?id=84564
I'm not sure if this is fixed but it no longer happens in the above case and is much harder to trigger in general.
Upstream.