The force was not quite with David Prowse, the man behind the mask, when he accidentally revealed the film's big secret in an interview with The San Francisco Examiner two years before the premiere of Empire Strikes Back.
"Darth Vader and Luke Skywalker are hooked up in a do-or-die light saber duel when Luke learns that Darth is, in fact, his long-lost father."
In retrospect, the actor has mentioned that he was only speculating at the time, and many of those who read the article reportedly thought the whole thing was a joke. The little interview never got any circulation beyond The Examiner, and maybe that's a good thing.
It was simply a very different time before the internet existed, ready to ruin everything for everyone.
What is your first memory of having the end of a film ruined by online trolls?