Comment 0 for bug 9863

You logout of Gnome. Later you login, but you get an error box stating that
gnome-panel could not start because it detected another instance of itself
running. Either the panel wasn't killed at logout, or it tries to start itself
twice. I suspect the latter as I see other apps (via the bottom of the Gnome
splash) occasionally appear to try to start twice.

Problem seems to be random and cannot be repeated at will.

Problem can be temporarily fixed by switching to a tty (by hitting ctrl-alt-f1)
and logging in and issuing a "killall gnome-panel".

This seems to kill one instance of the gnome-panel, thus clearing the error and
allowing the second gnome-panel instance to start normally.