I learned something new about IE9 today. Apparently, when visiting a site in the same domain as the computer, IE9 defaults to compatibility (quirks) mode. I am guessing that this is Microsoft trying to accommodate crappy intranet sites (cough cough SHAREPOINT cough cough).
So, if your site depends on standards mode, you might find that it looks fine in the real world but not in the office. I work remotely and this has burned me twice. Both times my launch celebration was cut short by complaints from colleagues in the office who saw broken pages.
Now I know.... And you do too.
Hat tip to Marjoe Bacus for letting me know about this "feature".