Continuity: 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: When Eliot Ness is at Jim Malone's house for the first time, Malone's collar is opened, then buttoned, then open again.

Continuity: In the scene where 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.

Revealing: When the four Untouchables are in Canada working with the Mounties, the smugglers are charged by a pack of Mounties on horseback. Despite the use of many tommy-guns, not one horse or rider is injured, even though they are riding wall to wall, with no spaces between them.

Continuity: 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: 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: 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: 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: In the scene when Nitti approaches Malone's house he checks the address in his match-book. It contains about five matches. 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 about ten matches in the book.

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: During the shootout on the rooftop, there is a point when Ness is above Nitti who then starts shooting. He hits 2 glass bottles next to some sort of bucket. However, in the next shot of Ness this bucket has simply vanished.

Plot hole: When Ness chases Nitti onto the roof of the courthouse, Nitti turns to fire back at him. He fires his .45 empty - we can see that the slide is back and the barrel is protruding - then runs a bit more, then turns to fire again. Anyone who knows anything about guns - like, say, a professional killer - would know when an automatic is empty.

Continuity: In the scene where 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.

Revealing: When Malone is shot in the alley, he turns around and a square bulgy thing protrudes underneath his shirt, obviously the blood squib.

Audio problem: Ness grabs a man who's trying to blackmail him and throws him out of the office. When he does, his voice changes abruptly, much cleaner, obviously overdubbed, and the sentence "You tell Capone that I'll see him in hell" is heard but Ness doesn't mouth it.

Continuity: During the Canadian border scene, the distance between the trucks keeps increasing and decreasing dramatically between shots.

Continuity: The girl that takes the bomb briefcase walks out of the bar, and stands by the exit door, couple of meters next to the street. When the angle changes to a back shot, she appears several meters behind the door, inside the bar.

Continuity: 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?

Revealing: 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: A man enters Ness's office and throws a manila envelope on top of a sheet of paper. In the close-up, the paper transforms into a folded newspaper and the envelope appears perfectly lined on it.