[Suzuran]

It happened but I don't remember which movie. It's not the often-quoted Star Trek instance - In that case, the woman was a first-time extra who was not aware that extras are not supposed to speak, even when directed to "act natural". In the case I am thinking of, they forgot to rope off a park entrance or something like that, and someone who regularly went there just did their usual thing and blundered into an active shoot.