I’ve run into multiple issues using gnome-globalmenu. I like it, but it doesn’t play well with others.
Its latest offense is that it causes the gnome screenlets manager to die on start-up with:
Attempt to add property GtkMenuBar::local to class after it was derived
Just removing globalmenu from the panel isn’t enough to get past the issue. You have to delete (or rename) /usr/lib/gtk-2.0/modules/libglobalmenu-gnome.so