"Don't know if it's that great, but it's my favourite:
In the Dark Knight, whenever The Joker is leaving the hospital as it explodes and then pauses to fiddle with the detonator when the explosions stop, that wasn't supposed to happen and Heath Ledger fiddling with the detonator was ad libbed and Nolan decided to keep rolling and include it in the film"
https://www.imdb.com/name/nm0778734/