Children of fixed elements should stay fixed... but no in IE8b2. Try scrolling and you'll see it move, but then try resizing your browser window. Very buggy behaviour.
On my site Party Ark, the 'lightbox' style pop-up boxes to enlarge the images don't have 'close' buttons any more - or if they do, they might be anywhere :).
You may also want to try out the document.createTextNode bug report page.
This div is 4000px high to force scolling.