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 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.
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.
Factual error: The film shows government agent Eliot Ness throwing Al Capone's right-hand man, gangster Frank Nitti, to his death from the roof of the Chicago courthouse in 1929. It never happened. Frank Nitti died of a self-inflicted gunshot wound in 1943.
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 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: 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 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.
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.
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: 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?
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.
Factual error: We see a British double decker bus on the streets of Chicago. The first Routemaster double decker bus was first introduced in 1956 so the existence of such a bus in the 1920's is impossible. Also, given its UK layout, designed for travel on the left side of the road, any passengers getting off would do so into the middle of the traffic.
Continuity mistake: When Nitti approaches Malone's house he checks the address in his match-book, and there are no matches on the right side. In a later scene in the courtroom, Ness takes a book of matches from Nitti, opens it and sees the incriminating address. This time there are matches on the right.
Other mistake: When Nitti (Billy Drago) kills Oscar in the elevator, not only is there not nearly enough time for him to write "Touchables" on the wall, but the writing looks like it was done with the aid of a ruler. If he had actually written it in the minute it took the elevator to reach the bottom floor, it would be smeared all over the place. (01:06:50)
Continuity mistake: Just after Ness sends his wife and daughter to a hidden place, he and Wallace are standing in the middle of the street, when a car approaches with Malone and Stone in it. They both get off the car and run to join Ness and Wallace, who are about five steps away. When the four agents go back to the car, they have to run about twenty steps, when they were supposed to be only five steps away from it.