Quick reference for myself, since I don’t see a good place to report this bug on the Linux Mint site, and I really don’t want to bother investigating this right now.
In the notification area (system tray) next to the clock, Mint 8 allegedly has a Fast User Switching applet. This applet, at least on my installation, doesn’t have an actual icon and often just “clones” half of the mintUpdate icon.
Removing said applet causes my GNOME panels to repeatedly crash and restart, either erroring out on Fast User Switching or mintMenu. The message box disappears pretty quickly so I don’t have a decent screen capture.
The easiest fix I’ve found so far is to trash the ~/.gconf/apps/panel/applets/%gconf.xml file from a failsafe GNOME session and then log back in properly.