Being John Malkovich (1999)

In the cult comedy film Being John Malkovich, there is a scene in which someone in a car throws a beer can at John Malkovich. This was not originally in the script — an extra had become drunk and belligerently decided to pull the prank. Director Spike Jonze enjoyed the bit so much he decided to leave it in the film. Probably the funniest part about all of this is that the extra reportedly ended up getting his SAG card and a pay raise for his work.