If the element that has an overflow has a child that has a position, then the child will act like position: fixed relative to its rendered placement and the scrollable region’s scrollbar will have no effect on the positioned children.
This affects both Internet Explorer 6 and 7.
The solution? Set the scrollable element’s position to relative (in CSS). Thanks to Alex Hudson, aka. so_solid_moo, for a pointer to the fix.
(Sidenote: So, one should be able to actually use this bug to emulate position: fixed in IE6, I think.)