The biggest guest faux pas:
Plus-Ones: Asking if you can bring a guest, when you receive an invitation without one is one of the biggest guest faux pas. It puts the host in an incredibly awkward situation. The best way to avoid this, is never to ask!
RSVP: Today, many hosts make replying to an invitation incredibly easy, with a reply set that is already stamped and ready to go. You simply need to fill it out, and stick it in the mailbox before the date listed on the card, however, 30-40% of guests tend to mail it in after the due date, or not at all. I try to mail reply cards back the day I receive the invitation to avoid forgetting about them.
[Photo courtesy of Ira Lippke of Ira Lippke Studios]