What is a famous line from the script of a movie that for months or years you always got wrong until finding out how it’s actually said? ~

Casablanca - The actual line in the film is 'Play it, Sam'. Something approaching 'Play it again, Sam' is first said in the film by Ilsa Lund (Ingrid Bergman) in an exchange with the piano player 'Sam' (Dooley Wilson): Ilsa: Play it once, Sam. For old times' sake.