Charlie's Gun
Just watched this a third time.
What's the colloquial definition of insanity? Doing something over and over and expecting a different result. Charlie's gun always jams at the crucial moment. Any sane person (or gangster) would toss that gun away after the first jam (or fix the faulty firing pin/mechanism). Not Charlie. He likes the grip. So his gun keeps jamming. And it always surprises him.
This is one of those movies that gets better with each watch.