A number of years ago a BA flight from Kingston Jamaica had two passengers who questioned why the Captain had given information about their destination, Gatwick, as they were going to Heathrow.
It transpired that as you board on foot in Kingston via the apron these two ,who were booked on Air Jamaica to Heathrow, had walked up the steps of the BA aircraft parked next door.
By chance their seat numbers were unoccupied and the crew did not notice two extras!
The CSD took the bizarre course of action by not telling the Captain and assuring the passengers that BA would provide transport for them from Gatwick to Heathrow. He ended up slinging trays at the back end of the aircraft after a disciplinary.