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 Canadian border scene, the distance between the trucks keeps increasing and decreasing dramatically between shots.
Continuity mistake: 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 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.
Revealing mistake: When Malone is shot in the alley, he turns around and a square bulgy thing protrudes underneath his shirt, obviously the blood squib.
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: During the meal, when Al Capone says he loves baseball, the man on the right holds a cigar with his hand. A shot later, from another angle, the cigar is on a plate and he is clapping.
Continuity mistake: 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.
Factual error: At the end of the movie, a pack of Lucky Strike cigarettes is shown on Ness' desk. The circle on the pack is red. Before America entered World War Two, Lucky Strike cigarettes had a green circle. The red circle was added with the ad campaign, "Lucky Strike is going to war."
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 elevator murder scene, in one shot there's not much blood on the wall, but in the very next shot there's much more.
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)
Answer: 'Vesti La Giubba', an aria from the opera 'I Pagliacci'.
jle