Sometimes actors go off-script while filming a movie, either intentionally or accidentally. It could be a last-minute idea, a joke, a way to cover up flubbing your lines, or just plain clumsiness. What's rare are those times that these moments are so good they ended up in the film.
In this video, Looper explains great movie scenes that did not go as planned, but ended up even better for it. -via Laughing Squid