The biographical narrative on Al Capone states he was born in Italy but raised in a Brooklyn slum. Capone was actually born in Brooklyn on January 17, 1899.
In the film the narrator states that three months after the St. Valentine's Day massacre Capone was indicted for income tax evasion. In fact, that indictment and conviction occurred in 1931, over two years after the massacre.
Name errors: The narrator says that Jack McGurn's real name is Vincenzo DeMora. His actual real name was Vincenzo Gibaldi. Also, the narrator says that Frank Nitti's real name is Francesco Nittoni. His actual real name was Francesco Nitto.
In the intro, it is stated that Herbert Hoover was Inaugurated on 14 February 1929. Actually, the date was 4 March. This date was changed to 20 January by the 20th Amendment - the so-called "lame duck" Amendment, ratified 23 January 1933.
2122 N Clark St faces West. The men were lined up to the left which would be the South wall. The massacre took place on the North wall.
Pay close attention to the massacre outside the church. The man firing the pump shotgun fires and fires, but he doesn't reload at all.
When Myrtle is throwing herself against the apartment door, the door frame and wall noticeably bend.
The narrator states that the scene in Schwimmer's living room occurs at 7:30 AM, but the time indicated by a grandfather clock is 7:20.
In the beginning the narrator refers to the era of the "black bottom" [a jazz dance]. Closed captioning reads the "black bottle".
Most of the Thompson sub-machine guns used in this film are model 1928A1's, which were not developed until the 1930's. More accurate is the Thompson M1921 used by Pete Gutenburg (George Segal) in the 1925 Hawthorne Hotel shoot-up.
Jean Hale's hair style is strictly 1967, not the least bit like 1929.
In the scene where the Gusenberg brothers pick up Aiello, a stage light is briefly visible reflected in the car's window as it drives off.
The opening credits show the SMC Cartage Company situated on 2122 N Clark Street, with Clark Street ending at a T intersection about a block away. Clark Street is actually a through street running from south of the Loop to the northern city limits.
In the scene where Peter Gusenberg is pushing the bar owner around, the narrator states the character was born in 1898 when in fact he was born in 1888.
Like with his brother Peter, the narrator states that Frank Gusenberg was born in 1902. In reality, he was born in 1893.
In the scene where Jack McGurn is showing the killers how to get to the Clark Street garage from the Wood street location, McGurn's spoken directions are correct but while he is saying "east on Webster" his finger is traveling north on Wood Street. Then while he is saying "south on Clark" his finger is traveling east on Webster.
When Capone is seen sitting in the Hawthorne Hotel dining room just before the big Moran shootout, his back is to the open reception area. No gangster, and few men in general, would ever sit in such a vulnerable position.