I occasionally get into a state in ML where the mouse click events stop working (and the Cmd-Tab UI doesn't show up). This persists until I kill "Dock". Then the mouse starts working again (and the desktop resets itself), but the dock and Cmd-Tab UI don't come back. (Cmd-Tab works, but doesn't display the UI.) I never figured out the magic thing to kill to fix this, and end up rebooting.

(This hasn't recurred since the latest point release, so it may be fixed now.)