This story takes place at a small airline operating out of a tiny aerodrome with no airport security. At check in, passengers exchanged their tickets for a boarding pass, but instead of printed paper boarding passes, the passengers received reusable laminated cards. The only thing printed on them was the logo of the airline. The passengers sat in the waiting area with their cards, and then, when the airplane was ready for boarding, exchanged their cards for admission to the airplane. This was perfectly legal and generally worked well for the airline.
One day, there were two flights boarding at once, and the busy customer service agents pressed a captain into service collecting boarding cards from the passengers for his flight. "The green cards are for your flight," she said. "Don't let anyone on who doesn't have a green card." Collecting cards is not rocket science, and the captain good-naturedly accepted the task and the instruction. The passengers were boarded and briefed, and the flight departed on time.
The first officer radioed dispatch as required, with time off blocks, time up, and a head count. There was a discrepancy between the dispatch numbers and the FO's numbers. after some recounting and a few radio calls back and forth it was discovered that one of the cards the captain had collected was not green, and the airplane was carrying a passenger who had bought a ticket for the other flight.
Why did it happen? I don't know for sure, but I suspect that the captain's medical certificate was printed with the words RESTRICTED TO DAYTIME ONLY. MUST HAVE TWO-WAY RADIO FOR CONTROLLED AIRPORTS. Such a restriction is placed on the medical certificates of pilots who are colour blind. They cannot distinguish between a red light and a green light to determine the direction of flight of other aircraft at night, or to receive instructions via light signals from a tower controller. And a red card looks the same as a green one.
That airline operated daytime flights only. Such a restriction might have helped the captain get the job initially, proof that he would remain with the company, not going on to the next level of commercial aviation. And it would explain why he did not seek out a higher job, despite his experience and competency. I never asked him.
The passenger was neither angry nor anxious at the unexpected diversion in his travel plans. He embraced the experience as an added bonus in his travel arrangements, an extra destination for the same price. And the little airline made sure he made it to his international flight in time.