At 1 hour 11 minutes into the film, The Chicago Daily Report newspaper is being read, front page headline "Valentine's Day Massacre". The right thumb of the reader is placed on the top right hand edge of the black newsprint. In the cut scene, his thumb is much further down the page.
In the opening scene when the mobsters are talking to Jack's father. He is shown holding $100 bills which are the current bills that are in circulation not ones from 1922.
The St. Valentine's Day Massacre took place on Feb 14 in Chicago- the middle of winter in a very cold city. In the street scene that takes place just before the massacre, people are walking down the street in Chicago without overcoats or winter clothing; one woman is wearing a sleeveless dress. There is also a patch of very green grass by the sidewalk.
The movie indicates McGurn was gunned down on Feb 14, 1936 (the anniversary of the St. Valentines Massacre in 1929) but it actually took place one day later, on the 15th.
In the opening boxing scene the audience has some Black people sitting in the front next to White people. Outside of a major boxing event Black and White people were segregated into separate sections.
At 1 hour 11 minutes into the film, The Chicago Daily Report newspaper is being read, front page headline "Valentine's Day Massacre". The second later cut shows the back page of the newspaper where golf tees are advertised with English prices of 1/- (on shilling) per box of six or 2/6 (two shillings and sixpence) for three boxes of six.
One of the characters refers to Moran as being Irish in a telephone conversation with him. Moran was actually born in St. Paul, Minnesota to French immigrant parents.
During a club scene with a live band, the drummer is seen using a modern "DW" (Drum Workshop) drum kit. The DW company was not founded until 1972.
There is a raid on a brewery by federal agents identifying themselves as "FBI". The term "Federal Bureau of Investigation" did not come to be used until 1935, which was years after the raid depicted in the film took place.
Opening scenes with the two mob guys at the cash register; the owner is holding $100 bills.
1 $100 bills were not commonly scene in places of business in the 20s.
2 If you look at the bills themselves, they are Benjamin Franklin bills circulated from1996 - 2013.