Ugh.. the rendering bugs.. when combining tables and divs... IE for the longest time had a bug where combining table and div elements nested, when exceeding a certain complexity, you'd just get a white screen... It was fixed in IE6 on XP, but not on earlier versions of windows.
That was a nightmare of support issues on a few apps for me.
Possibly my favorite bug ever: HTML has a weird relationship with whitespace; usually it doesn't matter much, except when it does: http://www.netmechanic.com/news/vol7/html_no1.htm
This is one of the few bugs I've encountered that still makes me angry to this day. I spent so much time trying to figure it out.