As you may know, Internet Explorer hates the Opacity property in CSS. Not so much a problem, as one can make use of filter:Alpha to accomplish the same effect.

Oh, but wait. Apparently that hides all overflow, no matter the overflow value, child position, or child z-index.

Looks like that was what was making one of my menus disappear at work. Slightly annoying, but manageable? I’d agree with that.