Comment 1 for bug 1152851

they say my window manager crashed:
http://ubuntuforums.org/showthread.php?t=1979689

you hit alt f2 and run
xfwm4 --replace
to fix it.

this solution was written a year ago. if this happens often in xubuntu desktop, this is a major bug. it kills the whole system and the user doesn't see why.