On an Easyjet flight, Nice to LTN, in Autumn 2002.
Everyone on board in good time, all in their seats and behaving themselves and the doors ready to be shut 10 mins before scheduled departure time. Since I was on my way home from a v. nice holiday, this was great as obviously going home is the hard bit.
Sat there for a further 20 mins, then Captain emerges from flight deck. Picks up the PA mic and announces, somewhat sheepishly, that he, the crew, us, and all our luggage are on the wrong aircraft. The one we were on had to be in Liverpool for a scheduled maintenance check that night - 'our' aircraft was the EZY one at the next gate!
To his credit, and by his own admission, he didn't want to hide on the flight deck to give us the bad news. To EZY's credit, we were all swapped over and ready to go in about 45 mins.
This is a true story, I kid ye not.