Comment 12 for bug 319726

I also observe this on my up-to-date Maverick. Another way to make it update again is to set the panel's position to a side of the screen instead of top, and then put it back on top (putting it on the bottom doesn't fix it). I've noticed other strange panel update problems, like zones that become white, or the taskbar fails to update (I can click on it to switch between open apps, but the app entries are not actually displayed). In all cases moving the panels to the side and then back fixes the problem.
It seems to frequently occur when I add/remove the external monitor.