The case
The most quoted line in the trilogy is misquoted almost every time. In the Cloud City duel, Luke shouts that Vader killed his father, and Vader answers: "No, I am your father." Watch the scene and there is no "Luke."
Impressionists, parodies and playground reenactments added the name because a quote needs a subject to make sense out of context. Forty years of retellings did the rest, and the version with the name is now the one people are sure they heard in the theater.
Why our brains do this
Quotes get edited for portability: out of context, "No, I am your father" needs a name to land. Repetition of the edited version overwrites the original.
Sources: Wikipedia — The Empire Strikes Back · Wikipedia — False memory: the Mandela effect
