re: Unscripted film shots or accidents that were kept in the final film (other than improv)

Thomas Mitchell playing Uncle Billy in It's a Wonderful Life. He is supposed to be drunk and stumbles out of the camera shot. A few seconds later a member of the film crew knocks over some equipment and it makes an awful racket. Suddenly you hear Mitchell from off screen yelling "I'm alright....I'm alright" as if he had done it. Jimmy Stewart cracks up a bit and Frank Capra decides to leave it in.