"I've detected another panel running, and will now exit" - ARRRRGH!

[Lee Revell]

I was watching a movie with mplayer and the screensaver apparently
kicked in which caused the screen to blank and the machine to become
unresponsive.  Nothing I did had any effect, even NumLock did not change
the LEDs.  Finally I hit "Ctrl-Alt-Backspace" which worked.  I got into
X again but as soon as I went for a menu the desktop froze again.  I hit
Ctrl-Alt-Backspace again and X restarted a second time, but this time, I
didn't get any panels, just an endless loop of "I've detected another
panel running, and will now exit" dialogs.  No combination of clicking
on the desktop could produce a terminal.  (Console switching with
Ctrl-Alt-F1 stopped working months ago, I can't for the life of me
figure how to get it back).

Finally, stuck in an endless loop with no way to get out of X, get a
working panel, or get a terminal, I had to hit the reset button.

What's the problem here?

Lee