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.