Whether the wedding has taken place or not, you probably already know who is going to cause problems! But for me my sister's boyfriend turned out to be the wildcard. They broke up/got back together/broke up/got back together several times before the wedding. Then I finally get to meet him at the rehearsal and realize he is a total scuzz bag.
My BMs were practicing putting my dress on (sister is MOH) and he comes into the hotel room and sits on the couch drinking a beer! While BMs have their hands/heads/etc stuck up my dress and various parts of my body are exposed! SIL had to physically escort him out of the room.
People were coming up to me all night asking me who he was and telling me that he had either given them his business card or inappropriately tried rubbing their backs.
Who was the guest all your other guests were talking about at the wedding? Or who are you most worried about misbehaving?