I’ve dropped out of a campaign because the GM revealed about session 2 or 3 that it was really a Cthulhu game. It had already been mis-sold as a campaign for mercenaries, so I had an utterly useless skillset (helicopter pilot, assault rifle) for the private eye stuff the first sessions entirely comprised of. Then Elder Signs turned up and I quit the game.
I was thoroughly hacked off with CoC and if the GM had been upfront that it was a CoC game, I never would have joined the campaign in the first place. The system was not CoC.
I’ve also been mighty miffed when I’ve signed up for a con game which was advertised as Cyberpunk, but all our characters ended up in a virtual reality ‘fantasy kingdom’ fighting D&D monsters.
On the other hand, I had terrific fun in a campaign where we were the bright eyed and bushy tailed village teenagers who eagerly signed up to join the King’s army and go off to fight the nefarious invaders from the kingdom next door - only to discover that maybe our side weren’t pure as the driven snow after all…