I accidentally closed a panel properties menu (I think the panel properties, but might have been global panel properties) and the whole panel instantly disappeared. No panel process existed, but when I tried to bring up another panel from a terminal session, I was told that another panel was running. After logging out of my session, gnome-session would not start anything until I deleted .gnome/session; then I got a chance to start over with a fresh session...
Might be fixed by new panel/session code?
This seems to be fixed now. (This was probably caused a bug in GDK where sometimes the WM_DELETE property would not be set on things started before the WM)