I'm not sure whether this is a consequence of the new CSS or not. We have a very visible publicity banner - it is shifted over to the side, and extends infinitely to the right. Perhaps as a consequence of the stretch placing the "close" action off to the horizon, the banner is also impossible to kill.
Apparently, this applies to IE7. I was originally going to call this as IE8, but it turns out I was actually on Compatibility Mode, for some reason. No repro on IE8, no access to any other browser at the moment. XP as OS.
Below is a screenshot of this unbalanced foe.