Arguably the most famous scene in Star Wars is when Darth Vader reveals to Luke Skywalker that
He is Luke's father.
This surprise was kept secret to most of the crew and even the cast during production, with Dave Prowse, the actor playing Darth Vader, delivering a fake line, the actual line being overdubbed. (Of course, Darth Vader wasn't voiced by him in the first place; all lines were overdubbed by James Earl Jones).
Like Mark Hamill relates in an interview:
But, for example, your big scene, one of the classic cinematic moments when Darth Vader divulges his true identity, is no longer a revelation.
It's such a great moment! The fake line that was put in there just to try and keep the secret was "You don't know the truth: Obi-Wan killed your father!" But as much as I enjoyed leaking false information, it was a wonderfully hard secret to keep because (Irvin) Kershner, the director, brought me aside and said "Now I know this, and George knows this, and now you're going to know this, but if you tell anybody, and that means Carrie or Harrison, or anybody, we're going to know who it is because we know who knows."
So how do we explain Dave Prowse knowing and divulging this same secret two years before the release?
And [David Prowse] offered a glimpse of a possible plot for the second sequel. Darth Vader and Luke Skywalker, the young hero in the first film played by Mark Hamill, are hooked up in a do-or-die light saber duel when Luke learns that Darth is, in fact, his long-lost father.
So exactly how secret was this secret anyway?
As DVK points out on Skeptics.SE, the newspaper article is probably authentic.