Having used his shotgun and stowed it in the getaway vehicle, Jackie wipes the car to remove any evidence but neglects to recover the weapon, leaving his fingerprints on it.
At one point in the film Frankie mentions a pub in Haverhill. He says it as "haver-hill", but anyone from New England will tell you the local pronunciation is "hay-vrul".
Around the 85-minute mark, Jackie kills Frankie, firing several shots from a revolver at point-blank range while Frankie is seated in the car's driver's seat. Afterwards, since Jackie wasn't wearing gloves, he wipes down the door handles, steering wheel and so on with a handkerchief. The last thing Jackie touched was the driver's door, just behind the nearside mirror. He obviously gripped the door there, yet he entirely missed wiping that spot.
Unless that wiping was meant as a synecdoche to imply he had cleaned everything, there are other areas he neglected. He didn't wipe the passenger side where he'd been sitting, nor the shotgun he left behind.
During the card game sequence the barrel gun pistol is unloaded — there are no rounds in its chambers.
Perhaps he wanted to prevent the weapon from discharging and killing anyone. Robbing the gangsters is one thing, but a killing during a robbery would inevitably draw in the homicide detectives.
