The Empire Strikes Back (1980) - This commonly quoted line, which Darth Vader was known to utter to young Luke Skywalker, was never said at all. The real movie line was "No, I am your father."
"I don't think we're in Kansas anymore, Toto"
The Wizard of Oz (1939) - The real movie line is slightly different, and less certain. When Dorothy arrived in Oz she said, "Toto, I've a feeling we're not in Kansas any more."
"Beam me up, Scotty"
Star Trek: The Motion Picture (1979) - The most well known "Star Trek" line, "Beam me up, Scotty" was never actually said in any of the Star Trek films. Instead Kirk said, "Scotty, beam us up."
The Silence of the Lambs (1991) - The creepy line uttered by Dr. Hannibal Lecter was quite different than it is remembered. He, in fact, said, "Good evening, Clarice."
"Mirror, mirror on the wall, who is the fairest of them all?"
Snow White and the Seven Dwarves (1937) - Every day the queen consulted her mirror on the wall, however, instead of the famous quote we remember, she called, "Magic Mirror on the wall, who is the fairest one of all?"