Titanic - "Leave the Blooper in ... It's Better!"
After handing Jack a dime, Rose steps back and drops her kimono to the floor. Jack, with a serious deer-in-headlights look (and I don't blame him), points and says, "Over there, on the bed ... uh ... the couch."
The scene as written merely said "Lie down on the couch." DiCaprio blew the line on the first take, but in editing Cameron decided to use the gaffe, on the premise that it was a more genuine reaction. (The Freudian slip also makes an amusing - if unintentional - connection to Rose's earlier reference to Dr. Freud.)