12 Biggest Lies Told In Star Wars Movies
Honourable Mention
Every Sith Apprentice Kills Their Master
The Lie
It's not exactly stated outright in the films, but a big part of Star Wars mythology says that part of the Rule Of Two suggests the nature of the dark side determines that every Sith lord knows their apprentice will try and kill them. It is suggested as a matter of fact, as it means the Sith will always grow stronger.
The Truth
Well, Darth Vader wasn't driven by that apparently singular compulsion. He did kill Darth Sidious eventually, but that was very much an emotional decision - at no stage did it feel during the films like he was actively looking to kill Palpatine.
Non-canonical texts (like The Rise And Fall Of Darth Vader) offer hints of it, but since the death of the EU, we can't accept them, and even though Palpatine dies by his hand, there's some issue with the suggestion Vader really WANTS to in the movies before that.