I searched and this wasn't reported elsewhere, but I received this error few moments ago when hovering over Alerts menu on top right side of navigation bar.
Browser:Firefox 3.6.8
And I must say this but, WOW, you have taken time to style even the error/alert/warning popups.
Thanks Kier. It's probably only a browser problem and a conflict with some extensions, because even after clearing all cache and cookies, the same problem comes up again.
Once you click OK, the error is gone.