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. (01:38:00)
Continuity mistake: During the scene where Ness kills the Italian mobster in the lodge near the border, the moment he shoots him the wooden rail behind has two sticks underneath, half a second later, when the mobster is flying backwards dead, the sticks disappear, and a second afterwards, when he is laying dead, the rail's appearance has nothing to do with the previous two.
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.
Other mistake: When the agent is shot in the freight elevator he is shot point-blank in the head. However, when Ness finds him there's not a drop of blood on his clothing, or a hole in his head.
Continuity mistake: In the final court case, as Eliot Ness escorts out Nitti, Ness's wife watches them leave and is wearing a brown and cream hat, with a brown jacket. When the case finally resumes and the guilt plea is changed, Ness's wife is again shown, except this time she's wearing a light blue hat with dark blue ribbon, a blue jacket, with a floral green and blue blouse.
Continuity mistake: When Malone takes the Untouchables to the post office on a liquor raid, he gives his shotgun to Stone and uses an axe to break down the door. When he reaches through the door to open it, the view switches to inside the room. Malone comes through the door with his shotgun again. There is no time for him to drop the axe and retrieve his shotgun. (00:35:25)