Status, as of 21 October 2011.

I have observed a persistent flaw in the operation of the LXDE
0.5.8-2 lxpanel function in Knoppix 6. I have commented on this in
Knoppix and LXDE forums several times previously, to no effect.
The flaw appears to apply to other LXDE distributions as well.

I have personally only observed this flaw in the context of
Knoppix 6, when using LibreOffice for some rather complex spreadsheet
operations, used always together with a browser on-line and using
some cut-and-paste data manipulations.

On closing LibreOffice after some of these spreadsheet operations,
I occasionally find that lxpanel no longer responds to mouse and/or
keyboard inputs. At such times there may remain icons on lxpanel,
representing programs including LibreOffice that were previously
in use. Also, at such times, the desktop responds to mouse right-
clicks, but the Knoppix screen for the most part responds only to
the Control-Alt-Backspace command, so far as I am able to determine.
No Alt-Tab, Ctl-Fn nor Ctl-Alt-Fn commands seem to have any effect.

A mouse right-click on the desktop space may bring up any of several
GUIs, but none of these offer any avenue to input an appropriate
corrective screen command, as far as I can tell.

I can easily reactivate lxpanel and regain mouse and keyboard
functionality with the Control-Alt-Space command with apparently
no ill effect to LibreOffice, which apparently shuts-down correctly.
I am unclear as to whether other programs, such as a text editor
still open at the shut-down of LibreOffice may have been affected.
Were it possible to do so, I would have chosen to attempt to close
all programs still open, before doing a full reset of X.

It would appear Ubuntu thinks this is an 'upstream' problem,
'affecting (only) three people', and one gets the impression the
LXDE forum is not close enough to LXDE's godfather to get any
corrective action initiated there, either.

I am open to suggestions, but Control-Alt-Backspace is the only
method of recovery of which I am aware.