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.
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: 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: It's about 50% truth and 50% fiction. Ness, Al Capone, and Frank Nitty are real, of course, but the characters played by Connery, Garcia, and Smith are fictional. Ness started out with 50 candidates for his 'Untouchables' force. This was whittled down to 15 finalists and from that he selected 9 agents (none of which has the same name as the characters played by Connery, Garcia, and Smith). It's true that Capone was convicted for tax evasion. He was sentenced to 11 years in prison, but received an early release because he was in the last stages of syphilis. He died shortly after being released from prison.
raywest ★
Capone lived for 8 years upon his release from prison. The 8 years being slightly longer than the actual prison time he served, which was just over 7 years. He died, having the mental capabilities of a 12-year-old.