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

I couldn't begin to tell you the movie but one my dad showed me YEARS ago when I was a little kid was some old black and white film and there's a large gathering somewhere (almost like a school cafeteria or restaurant or somethign) and just before a man is about to pull a gun and shoot some other dude there's a kid extra sitting in the back that plugs his ears up knowing what's abotu to happen.

Not really all that "cool" but I suppose it would fit the criteria.