I heard this one today... A groom worked for a very important and respected boss, and invited him to the wedding. The boss got to the reception, and he and his wife were looking around for a place to sit. They didn't see anything other than a little table for two, so they just sat there.
Of course, this was the sweetheart table, which wasn't really popular until a few years ago, so he'd never seen one before. He and his wife sat there for the whole reception, and since he was the groom's boss, and a very important and respected boss, nobody said anything to him.
The next day the important and respected boss realized what he had done, and was totally mortified.
This is one case where a "reserved for the bride and groom" sign would have been a good idea.