.
There is a fairly widespread bug which appears most likely to be in
lxpanel which affects a number of Linux distros, probably described
as well for all in Ubuntu Launchpad Bug 800297 at

https://bugs.launchpad.net/ubuntu/+s...el/+bug/800297

I think this bug has plagued Knoppix 6 for a long time. I get around
this bug with Control-Alt-Backspace, and don't think I have lost any
'work' in progress that I can recall.
However, I think this is an unhealthy aberration which should come
under greater scrutiny culminating in some corrective action.

There is an alternative to lxpanel called Tint2 which may not have
the problems of lxpanel which some Linuxers are trying out. See

http://code.google.com/p/tint2/

I'd be interested to know:
(1) If others on this forum have observed the lxpanel-freeze bug;
(2) If anyone has a really 'bad' experience as a result of this bug;
(3) If any of you found work-arounds or solutions to this problem; and
(4) If anyone has tried out Tint2, and if so, how did that turn out.