I've run into a problem with a layout that seems to work fine in every browser I've tried it in except NS 7.1 on the mac.
Basically I have a centered percentage width div that I want to appear against the div above it. Netscape puts a margin above the div. If I put a top border on the div the problem goes away. The problem also goes away if there are no block level elements inside the div, for instance if I get rid of the <p> tags in the attached example.