Anybody who doesn't know the twist of ESB doesn't deserve to be alive. That one goes right up there with "it was his sled" (Citizen Cane), "Soylent Green is people" (Soylent Green), and "the planet of the apes is Earth" (Planet of the Apes). If you don't know those things, you've been living under a rock. :P

That said, I once accidentally spoiled Harry Potter five for my friend's younger brother... It was well over a year after the book came out, but I still felt awful.