Just out of curiosity, what's the rudest thing you've seen a guest do? Doesn't have to be at your wedding.
I'll throw mine in. During a friend's wedding reception they ran out of alcohol (not an ideal situation I realize), but some guests at our table (I didn't know them) went to the store and bought some beers to bring back to the table. I personally found this ridiculous and totally uncalled for.
Anybody else?