In the scene where Leo uses the Thompson sub-machine gun he
should have had to reload at least 6 times. Assuming the gun is a 1928 model the rate of fire is 700 rounds per minute and has a 100 round can of ammunition. The gangster walks into the bedroom and fires for 5 seconds for a total of 58 shots fired, Leo takes his gun and fires at the window for 20 seconds for 233 shots fired, then Leo fires at the car for about 20 more seconds for another 233 shots fired. That is a total of 524 shots fired from one Thompson with no reload.
Verna dials 911 when Tom Regan comes to her apartment, on the night of the hit on Leo's house. In those days, you had to dial 0 and ask for the police if you wanted them quickly, but then again, there were operators by the hundreds available at the spin of a rotary dial.
When the Dane goes to Verna's apartment, Verna pulls a gun out of her purse on the bed and points it at the Dane. He then slaps her hand with the gun, but the sound of the slap is clearly before the visual.
Caspar repeatedly refers to Bernie as "the Schmatte" - Yiddish slang for "a worthless person". Bernie is Jewish, but an Italian gangster would never use a Yiddish word to refer to him. He would use an Italian epithet, instead.
After Tom loses his hat to Verna, he goes to her apartment and spends the night (Apartment #3). Yet, the next morning in the very next scene they are both at Tom's apartment, a different building, the Barton Arms, the same morning that Leo dropped by.
At 20 minutes into the film, when Tom Reagan pours his third drink at the bar, he never drinks it. When he walks away with the "full" drink in his hand, the glass is empty. When he gets to the dressing room in the next scene the glass is full - almost to the top. Reagan only poured briefly at the bar, about one shot, not enough to possibly fill the glass.
At 00:36:10, when Tom talks with Verna, she's holding cigarette between the forefinger and the middle finger. At 00:36:18, she's holding it between the thumb and the forefinger. And back to the start at 00:36:25.
The goof items below may give away important plot points.
When Tom is picked up by TicTac and The Dane before they go looking for the body in the woods the street changes between shots - a black car appears beneath the High & Sons sign in one shot then disappears when TicTac swerves his car onto the pavement.