Very easy to illustrate:
var newElement = new Element("div");
$(document.body).insert(newElement);
assertEqual(document.body, newElement.getOffsetParent()) // fails in IE6 - it returns the HTML element instead
This causes all sorts of really fun to track down problems down the road.
The fix is pretty straight forward - the only odd part is that element.offsetParent will actually return the correct BODY element up until element.setStyle({ position: 'relative' }); is called (inside the IE-specific wrap for getOffsetParent). Something about relativizing the element causes it to have its offsetParent property to the HTML element instead.
