Flying DL 745 DTW - SFO this morning. Full 767. With everyone boarded and the door closed, a woman walked to the front of the plane and said that she is supposed to be flying to SAN. Gate is called, bridge returned, door open, and she deplaned, then we shut the door and pushed back.

It seems surprising that the boarding pass scanner wouldn't have caught this, and then that on a full plane someone wasn't assigned to her same seat. Can this really happen?