Just a short note on Opera http://www.opera.com/
It has occured to me several times now that this website was missing the whole background colour in Opera (the background was all gray).
Effectively, the <div> that serves as the main container already ended at the header!
The cause of this has of course always been an error on the human side: me forgetting to escape < properly with <.
So if you ever notice your background missing or a div ending earlier than it is supposed to, check your website for stray <!
I must say, Internet Explorer and Firefox cope much better with this.