I've ran into one of those fun things that CSS can do. I have a layout that has a header with a menu and a couple of columns. The header pokes down on the right hand side. That area poking down contains the menu. To fill the gap that that makes, the left column is positioned higher than the right column.
I was having problems with it consistently laying out properly and went and tried it in a table. I got it looking and working perfectly in Firefox, but IE considered the mouse clicks on a menu button to be going to the table. So I had to rethink it all in DIVs and to float each column. I finally got that going in both browsers after IE would make them vanish (note: I think the container needs a width to prevent that).
Now it looks good in both Firefox and IE, but Firefox is acting like IE was with the table layout. Sigh...
Warning: The first Internet Explorer engineer I meet, I'm going to ring his neck!