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

not sure if it meets the criteria, but the scene in Close Encounters of the 3rd kind, when the aliens come to abduct the kid - all of the things that "come alive" in the kitchen were scripted, but the actress was not told it was going to happen - all of her panic and screaming is genuine terror.

like i said - not sure if it meets the thread criteria, but thought it was a pretty cool scene. (also my mother met Dreyfus at Bel Aire Mall in Mobile during filming since part of the film was done in Mobile - she was 5 months pregnant with me at the time)