When Tom throws the glass at the mirror in the woman's washroom, the cracks in the glass change between shots.
After the Sons of Erin club blows up, a burning body flies out onto the street. In the subsequent shots, it is gone.
Tom's whiskey glass on Leo's desk disappears after he confesses to Leo.
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.
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.
At 0:16:48 when the boy pulls the wig off the dead man, the dead mans bottom lip moves
The US flag in the mayor's office in the 1920s appears to be a 50-star flag.
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.
After The Dane punches Tom to the floor in Caspar's house and threatens to beat Mink's fate out of Tom, the line, "Is this how you taught Drop his story?" from Tom is dubbed over his unmoving lips.
When Johnny Caspar is being driven up to Tom Reagan's apartment building he is talking about how to get a perfect shave but his mouth is not moving.
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.
After Tom shoots Bernie, he leaves his fingerprints all over the gun.
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.