Factual error: In the warehouse scene with the wooden crates with maple leaves, even though a correction has been submitted saying a maple leaf was valid, the real mistake is the type of maple leaf used. The film shows a modern graphic style of leaf that did not come into use until the 1960s and appeared on the new Canadian flag in 1965. In the time of Capone, the maple leaves used to depict things Canadian looked like actual maple leaves and not like a stylized graphic.
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.
Factual error: The film shows government agent Eliot Ness throwing Al Capone's right-hand man, gangster Frank Nitti, to his death from the roof of the Chicago courthouse in 1929. It never happened. Frank Nitti died of a self-inflicted gunshot wound in 1943.