Many years ago, travelling to Cyprus from Luton. In the departure lounge, I noted 2 flights of the same airline and with similar callsigns ('540 and '542) were departing to Cyprus about 15 min apart.
When we got on board al seated, the cabin crew did repeated head counts. It then transpired 4 passengers (2x2 not known to each other) from the other flight had been allowed to board our flight.
It can work the other way though. Departing Heathrow for Geneva and being 'bussed' to the aircraft, the attendant at the gate called for 'passengers to Zurich'. No-one answered until someone said we were trying to get to Geneva.