It's a bug I've seen playing solo without any boxing software loaded, so it isn't a problem limited to multiboxers. The only way I've found to fix it is /console reloadui. If that doesn't fix it, exiting to the desktop and relaunching WoW is the only other fix.

Fortunately, it is exceedingly rare to see this. I think I've hit the bug maybe 4 times in the last 6 months.