IE and italics
Ever seen IE/Win ruin a layout just because a few words are in italics? I have. Just the other day I was pulling my hair out over this bug, which decided to show itself on some pages of a recently released client site. At certain window sizes, the main content container would drop below the sub menu, only to pop back up if you made the browser window slightly narrower or wider. Ah, the wonderful magic of Internet Explorer.
Luckily, I found Bruno Fassino's excellent dissection of the problem at Position is Everything: IE and italics. The article thoroughly explains what causes the bug to occur as well as ways of preventing it from appearing. Thanks to that, I still have most of my hair left, and the problem is taken care of in IE 5.5+.