Continuity mistake: When Ness is "escorting" Nitti off the roof of the courthouse, watch the proximity of the two to the rooftop door. When the camera cuts from the edge of the roof to Nitti and Ness and then back to the edge of the roof, you can see that they went from close to the rooftop door to really far away from it.
Continuity mistake: When Eliot Ness is at Jim Malone's house for the first time, Malone's collar is opened, then buttoned, then open again.
Continuity mistake: When Ness and Nitti are fighting on the roof of the courthouse notice the sky. It is clear, then cloudy, then overcast then clear, etc.
Continuity mistake: When Ness and Malone are shooting at a gangster car on the bridge coming towards them, they blow out the left headlight. The shot changes to them firing and back to the car. When it cuts to the car for the final time, they shoot the left headlight out again.
Continuity mistake: In the train station, where Ness is trying to catch the baby carriage, he shoots a gangster when the man makes a charge from behind the pillar. The bullet clearly hits the gangster's left arm (at least blood squirts from there), but when in close-up the gunshot wound is in his chest.
Continuity mistake: When the four untouchables charge, Agent Stone and Agent Wallace run to the bridge off their horses, whereby Agent Stone gets shot in the left arm. Later, back in the police HQ, his left arm shows nothing of a bullet passing through or bandage.
Continuity mistake: When Al Capone has sent hit men to murder Malone, he is shown going from one room to another, alternately, from outside and then from within his apartment. When the shots are taken from outside, the camera pulls up tight to the outside window. You can see the cameraman on the crane.
Factual error: In the warehouse scene with the wooden crates with maple leaves, even though a correction has been submitted saying a maple leaf was valid, the real mistake is the type of maple leaf used. The film shows a modern graphic style of leaf that did not come into use until the 1960s and appeared on the new Canadian flag in 1965. In the time of Capone, the maple leaves used to depict things Canadian looked like actual maple leaves and not like a stylized graphic.
Revealing mistake: When they are on the bridge between USA and Canada one of Capone's men is shot (in the breast) by the rail of the bridge. Seen from outside the bridge you can clearly see that the stuntman is not falling, but he is jumping off the bridge.
Continuity mistake: In the scene where George and Wallace enter the elevator with Nitti, the elevator doors are facing the interior of the police station. There is no visible second set of doors on the back (exterior side) of the elevator, or a second set of buttons. Nitti turns and shoots George and Wallace, leaving their bodies slumped against the back (exterior side) of the elevator. By the time the elevator arrives at street level, it has done a 180 degree turn, with the doors then facing the exterior of the building, where Nitti can exit directly to the street. The doors are not just hidden when closed, because the bodies are then slumped against what would be the interior side of the elevator, and the only set of buttons is now on the exterior side.
Continuity mistake: When Ness is chasing Nitti across the roof (just after he has blown his hat off) there is a close-up shot of Nitti running towards the camera (the camera is moving backwards) and Ness can just be seen in the background running after him. He then climbs over a wall and runs down some steps. As he climbs over the wall the sun is shining on his back and when he is shown running down the steps on the other side of the wall the sun is now shining on the building from the opposite side.
Continuity mistake: After the death of Wallace, when the three remaining Untouchables are gathered, Stone says to Ness that he requests to stay (or something similar). Then he walks off several steps away from Ness and passes into the kitchen. The shot changes and suddenly he's only a step away from Ness.
Continuity mistake: The hand written note with the "I'm very proud of you" is different between the two times it's shown, even though it's meant to look like the same paper. The second time the "V" is capitalised, the writing is not slanted, and all the rectangular foldings from before have disappeared.
Continuity mistake: Ness has been leaning over Malone who was laying on the floor of his apartment shot up and bleeding profusely. Some blood shows up on Ness' overcoat but when they go to the train depot there is no sign of blood anywhere. Since there was no time to change where did the blood go?