Continuity mistake: When Lt. Snyder breaks the phone booth window with his gun and points it at Hooker, Hooker's hat falls off. Yet in the next shot over Lt. Snyder's shoulder you can see that Hooker's hat is back on his head.
Factual error: When Lonnegan brings the briefcase full of money to the cage for his big bet, all of the 100s in the case are 60s federal reserve notes, which obviously did not exist in the 1920s or 30s.
Revealing mistake: After Hooker escapes the attack by Lt. Snyder in the phone booth, he runs down an alley. On a cross street in the distance, a modern sedan and large white truck drive by.
Continuity mistake: When Hooker meets Gondorff for the first time, he puts him in a shower, fully dressed. When the camera cuts to a front view of Gondorff, as the shot goes back and forth between him and Hooker, Gondorff's right pant leg alternates between being soaking wet and completely dry several times.
Continuity mistake: When Lt. Snyder meets with Polk, the FBI agent, they both sit down and Snyder makes a comment about Polk's "nice office," which is really an empty factory. Polk gets offended immediately and from an over-the-shoulder shot you can see him rise from his chair. The camera switches to a shot of his face and he's calmly sitting down again, then rises up in anger. He stands up twice.
Factual error: When Hooker comes out of the diner there is a billboard advertisement to the right. It is for Ezra Brooks bourbon. Ezra Brooks brand wasn't created until 1957.
Other mistake: Why is the cashier sitting behind bars in the mock-up bookies when he just steps out of the side to go and fetch Gondorf? If anyone wanted to get at the cashiers they could just walk around the side where there is no door.
Character mistake: At Duke Boudreau's bar, the potential recruits for the sting are told, "Gondorff is setting up a con on the North side". Later in the film, Agent Polk tells Lt. Snyder, "Gondorff is running a con on the South side". The actual address for the betting hall is not given, but it is just down the alley from Klein's, which does have a South side address.
Factual error: The Helvetica typeface, used on the 43rd Street 'L' platform is not correct for the period of the film.