This happened to a friend of mine.....back in 2000

Everyone boards the plane...taxi out to the take off strip....FA goes on the mic & says hi folks welcome etc....states where they are going, how long it will take etc....normal stuff....

right away....some passenger pressed the button for the FA.....FA comes over....says hey I am on the wrong flight....they turn the plane around, disembark this person & re-taxi out....the whole process delyaed the flight by about 4 hours.....longer than the fligth time from Detroit to ATL....I was at the airport waiting to pick up my girl friend....I was NOT happy with that SOB......ruined hundreds of passengers schedules for just 1 guy....