I was working on CSS layout written by someone who thought that the size given to an element is size+padding+border like in IE5 and not only the content box as explained in official specification. It was written only a few month ago so he did dirty hacks to make it look well in IE7. It took me several hours with FireBug to track down the source of the problem and by the time I realized it I was really annoyed.
If you open site with "floating" CSS written for IE5 in Firefox the boxes just do not have enough space to fit and fall down the page. If you open it in IE7 it looks nice as IE7 lets the borders overlap so it looks almost correct. For someone as inexperienced as me it was hard to note.
