It was a bug in the way IE handles the overflow tag under certain circumstances, ie interaction between relative positioning and absolute positioning. What's interesting is that it causes an overflow in ie so it probably qualifies as a potential security bug.

There are plenty of css issues in IE, in this site alone I've run into and had to create workarounds for:

auto margin handling, float peekaboo, expanding boxes,. incorrect width calculations on dhtml (This one really bugs me), line height and replace miscalculations, semi random doubling of float margins, and apparant phantom boxes. All of which are in the general css spec, but have problems in IE.