The funny thing (or not so funny) would have been if the flight was not full & no one else was assigned that seat. This woman would not have known she was on the wrong plane till she got off & IAD. I can just imagine the blame going back & fourth. She would have blamed DL for letting her board the wrong flight (and she would be right) but DL would have blamed her for being a dumb b... (& they would also be right). Takes 2 to tango