During a scene set in 1930, when Elliot Ness is listening to "Amos n Andy" on the radio, you can hear a studio audience. "Amos n Andy" didn't perform before a studio audience until December 1936. The broadcast we hear is from 16 November 1952.
The Chicago flag that appears throughout the movie did not exist in 1929-30. The 4 stars on the flag represent Fort Dearborn, The Chicago Fire, The Columbian Exposition of 1893, and World's Fair of 1933, which hadn't happened yet.
The maple leaf has been a recognized symbol of Canada since the 1850s. However, the maple leaf on the liquor crates during the raid is the stylized 10-point leaf designed for the modern-day Canadian flag, which first appeared in 1965.
Towards the end when Capone's thug is holding the book keeper at gun point, the 1911-type pistol he is holding has an external extractor right behind the ejection port. The 1911 originally had an internal extractor, whereas the external extractor is a modern invention recently added to the 1911-style pistols.
At the end of the film, as Ness cleans out his desk, a pack of 'Lucky Strike' cigarettes with the white & red logo are visible. "Lucky Strike" cigarettes had a signature dark green pack that wasn't changed to white until 1942 (the change was made because of the need for a 'modern' design, with a marketing campaign claiming the copper used in the green color was needed for the war effort, and if anything, the "Lucky Strike has gone to war!" campaign meant Camel and Chesterfield, not the Axis).
When Ness says the line, "You tell Capone... I'll see him in hell," his teeth are firmly clenched, and his lips are not moving. There is also a clearly audible difference between this "looped" line and the rest of the scene.
When Ness goes to Malone's apartment for the first time, Malone moves towards a bookcase. The boom mic is reflected onto the glass front of a police officer's picture on one of the shelves (0:24:40 into the DVD).
The judge presiding over Capone's trial orders that the jury be switched with the jury in a divorce case being tried in another courtroom. Capone is on trial for federal tax evasion, while divorces are state cases, so there could not have been a divorce trial taking place in the same courthouse as Capone's trial.
At the Canadian border, Ness, the other three Untouchables and the Captain are standing in a line of five. We then see the captain leave the line after he gives orders. When the camera pulls out, however, we see that the line the five of them were standing in is still intact. Someone on horseback is moving in the back, but unless someone took the captain's place, it cannot be the captain.
Ness's first bust is unsuccessful; he pulls an umbrella out of one of the Canadian boxes. When he first opens the umbrella, it has packaging straw on it. After a brief photo from the cameraman, the packaging straw disappears.
When Capone is replying to the interviewers saying, "I'm responding to the will of the people," there is a certain amount of shaving cream on his face. When we next see his face, a moment later, there is more shaving cream than before.
Oscar goes down in the elevator a few meters before the corridor turns right and after a flight of stairs. But when Elliot Ness realises something is wrong he runs down these stairs. Police Chief Mike Dorsett goes into his office at the end of the said hallway and looks out of his window which is situated on the line of the corridor and sees the assailant who killed Oscar and the witness. This mistake in essence means that the lift shaft moved 100 yards along the line of the corridor.
During the border shootout, Ness chases a bad guy to the cabin. He then tosses a grenade onto the roof which rolls then falls to the ground. Bad guy sees it - stationary and smoking. Cut to front-of-cabin angle and the grenade is bouncing on ground and explosion happens next to grenade.
At the bridge shoot out the perspective from Eliot's binoculars change. At first it's wide angle as he follows a car across the bridge. It changes to a close up as the bootleggers arrive to start the deal. The binoculars being used don't appear to have an adjustable field of view.
When Capone's man comes to Ness's office to bribe him, Ness throws the money envelope back at him. The envelope bounces off the man's shoulder and continues off camera (presumably to the floor). Less than two seconds later, however, Ness picks the same envelope up from his desk to push it and the man out of his office.
When the bad guy waits outside Malone's house before Malone gets killed, he checks the address written in his match tab. All the matches at the right end had been used up. Later in the court house Elliot uses the same tab of matches and finds Malone's address on it. This tab has almost all the matches on the right end. The handwriting on both the tabs are also distinctly different.
When Ness fires his gun, it changes from a M1911A1 to a Star Model B. The Model B is the Spanish clone of the M1911A1. The Model B is chambered for the 9mm Parabellum round (which is more reliable as a blank) while the 1911 is a .45 ACP. The 9mm was more reliable up until just after The Untouchables was finished.
Frank Nitti falls to his death into the car below, and Eliot Ness says "He's in the car." In the next shot, the camera is moving slowly toward the damaged car and Frank's body. If you look closely, you can see the camera crew gradually reflected in the car, as they walk toward the car from behind.
The film shows a bridge over a small river in apparently Western high-desert terrain. The bridge has no customs-immigration stations. In fact, there is no river (or water) boundary between the US and Canada between Lake of the Woods, MN and Puget Sound on the Pacific Coast. Bridge boarder crossings in Northern Minnesota and Michigan cross much larger rivers, connect two towns and are in forested areas.
In the movie Ness is portrayed as married with children. In real life, Ness was a young bachelor living with his parents when he was hired as a prohibition agent and used political/family connections to get his Chicago assignment.
In the roof top shoot out, when Ness falls off the roof to the ledge, a shot of Drago's pistol shows the slide mechanism is locked back, indicating it is out of ammunition. After Ness takes a shot at him, Drago is running away and tries to shoot back and you hear the hammer click three times. In pistols of the type he was using, with the slide locked back, the trigger and hammer mechanism are disabled.
In the opera house scene, first, Capone gives reporters an interview complaining of his treatment by Eliot Ness. He then attends the opera, watches Enrico Caruso perform and, subsequently, enjoys a champagne celebration with him. Except that Eliot Ness started to investigate Capone in 1929, and Caruso died in 1921.
A street shot of Malone's house shows the crossroads are Racine and Harrison. Harrison is an East/West street and has a designation of 600 S. The address of the house shown in the film is 1634 Racine. More accurately, it should be listed in the upper 500s or lower 600s, as in 602 S Racine. Or alternatively, the cross streets should have been shown as Racine and W 16th Street.
When the photographer takes the shot of the four untouchables seated at the table, his flash is positioned to the right of the camera lens, this would cause the resulting shadows to fall on the opposite axis. But the shadows in the photo indicate the flash was positioned to the left of the lens. When we again see the photo towards the end of the film, the shadows have moved and are now indicating the flash was placed directly over the lens, revealing that it is a completely different photo.
The prosecutor is called a "district attorney." District attorneys are state prosecutors, but federal crimes, such as income tax evasion, are prosecuted by federal prosecutors, not state prosecutors. Furthermore, Illinois has state's attorneys, not district attorneys.
After discovering that his case is a lost cause, Capone's lawyer switches his plea from not guilty to guilty, despite Capone's objection. A court cannot accept a guilty plea over the objection of a defendant, however, there's no evidence that the court DID accept the guilty plea.
Although the name "George Stone" does not translate to "Guiseppe Petri" in Italian, the character didn't necessarily directly translate his name. The Petri part - "Stone" - is correct, however "George" in Italian is "Giorgio". Guiseppe is actually "Joseph".
While the judge at Capone's trial should have allowed the defense to examine and approve the new jurors when the jury was exchanged, the defense objected and was overruled, so this would be a point for appeal. Also the jury switch is almost exactly how it happened in real life, exception that is was prior to the trial Capone was found to bribe the jury.
When Capone's man Overcoat Hood makes his report that Ness got the shipment of booze, not only is he not disheveled in any way, but he apparently made a journey of several hundred miles out of the wilderness of the Montana/Canadian border all the way back to Capone headquarters in an inordinate amount of time.
When the knife man is walking along the ledge outside the apartment, all the windows appear to be closed due to the reflection of the light. The last one, in the kitchen, first appears open and you can hear Malone pouring whiskey. The light, however, still reflects on the window indicating a closed window.
At the finale of the courtroom scene, Capone is seen becoming violently angry over the verdict, even punching his attorney. In reality, Capone accepted this verdict calmly, while meekly proclaiming to the press that he was innocent. Actually, Capone, while often violent and unpleasant when dealing with competitors and those inside his organization, was very protective of his public image as a genial, "misunderstood benefactor" of Chicago, and took great pains while in public (and when dealing with the press) to appear refined, polite, and well mannered. A public outburst in the courtroom (especially in front of the press) as portrayed in the film would have been totally uncharacteristic of him.