Read a bugzilla diff mail, or some equivalently long/wide message. Shrink your window (eg. laptop) so that it's smaller and the content area needs to scroll, you get two sets of scrollbars! This is really confusing for the user, it looks like content has been clipped, AND once you figure out that the scrollbars are there it's still hell to see the whole message.
Phil just filed a similar bug - http://bugzilla.mozilla.org/show_bug.cgi?id=18492
Thanks Lisa. I'm just going to mark this one as a duplicate. *** This bug has been marked as a duplicate of 18492 ***
Mark it a verified/dup.