Deliberate mistake: When the two assassins enter Leo's bedroom and try to gun him down, the one on the right has left his tan overcoat open. He's the one that escapes from Leo initially. Shortly thereafter, Leo guns him down, firing up into a second story window. At this point we see that the assassin's coat is now buttoned up, probably to conceal the numerous squibs used in his death scene.
Continuity mistake: When Bernie pays a visit to Tom after he decided not to kill him, Bernie has a pistol lying on the small table next to the armchair he is sitting on. When Bernie is talking first the gun is white, but when the shot changes to Tom and back to Bernie the pistol somehow turns into a black one.
Continuity mistake: Before Leo comes to Tom's place to look for Verna we see through a mirror that Tom is sitting on his bed. In the next shot, when the camera is directly on the bed the blanket is arranged differently, and Tom is sitting much more towards the edge than in the shot before.
Continuity mistake: When Tom gets back to his apartment after the dressing room scene he receives a phone call regarding his betting debts. When he hangs up Bernie appears in the chair opposite him although one could see the empty chair when Tom walked in, and at least the hat on Bernie's knee, if not more of him, should have been in the picture.
Continuity mistake: The scene where Leo comes to Tom's place to ask about Verna starts with a shot on Tom's hat lying on a cupboard. There's a clock next to it that shows 4.30, however, the clock on the wall behind Tom when he talks to Leo shows 3.20. After Leo leaves Tom goes back into the bedroom, and the alarm clock on the bedside table shows 5.20.
Revealing mistake: When the club Sons of Erin gets attacked by the police the whole place set on fire by a grenade, and even a brightly burning victim comes flying out. However, the fire dies within a few seconds obviously to allow the patrons to attack the police with automatic guns from inside.