Passengers aren’t supposed to be able to board the wrong plane. Boarding passes are scanned at the gate. There’s a manifest — airlines know how many people are supposed to be onboard. When passengers are in the wrong seat, there’s usually another passenger assigned that will say something. But it happens. Occasionally one person, or a couple traveling together, wind up in the same city.
Perhaps for the first time ever a majority of people on the plane were flown to the wrong city.