Mine would have to be the float and margin bugs…
If you float an element, and then specify a margin for it, it will double the margin.
The solution to this is to add display: inline to the element. This will stop the double margin, and all other browsers will ignore it because only block level objects can be floated.
I wish I’d known that I’d be spending hours trying to fix a bug in IE 6 over and over again for years on end and I really would have been happier delivering pizza.