Getting on wrong plane
 

It has happened to me. Back in 1985 I was booked on a Saudia flight from Riyadh to London Heathrow. It was a a particularly busy time of year so an additional flight was put on to supplement the scheduled one. I can't remember which of the two I was allocated to now, but I seem to remember the flight numbers were very similar, either differing by 1 digit or a suffix letter. Anyway, departure times for such flights were scheduled for on or soon after midnight. Due to a sandstorm all flights were delayed leading to some confusion, and when the flight(s) were announced, I duly boarded what I believed was my flight. It was only when I was challenged by another boarding passenger, who claimed I was occupying their seat, that it transpired I was on the wrong flight. So as well as my carelessness, the ground staff and cabin crew had failed to prevent me boarding the right aircraft. Thankfully all was resolved and I eventually reached London, albeit a few hours later than scheduled.

Years ago, when AirUK had the F27, several aircraft would be parked on the same bit of pier (no airbridges). I was operating the flight to Leeds and when all the pax were aboard it transpired that that we had the pax and bags for Teeside and vice versa. The easiest solution was to swap the flight deck and cabin crews around, which is what we did.

Circa 1985 Mrs A to be and I were flying LUT - AGP with Monarch. There was also a Britannia flight doing same route at same time.

Boarded the 757 and was idly watching a (Virgin?) Viscount being started on next stand when it became apparent that CC were having a 'flap'. Repeated trips up and down cabin counting heads, comparing notes and conversing with dispatcher. Eventually there was a request to check tickets etc and a young woman down the back was deboarded and went back through the terminal to board the BY737 further down the apron.

This is on the beeb today - not sure if it is recent or not...
BBC News - Passenger put on the wrong plane to the wrong country

The report is listed today and has this text:
The pax gives a lucid story of what happened. He sounds very laid back and reasonable about it no mention was made of his having to pay for the error and the question was not asked. Sounds like it was sorted out well.