Al Capone's scar was not caused by broken glass from a window, but by a knife wound in an argument between he and Frank Gallucio over a remark he made to Gallucio's sister Lena at the Harvard Inn on Coney Island in 1917.
In the movie, the last time Frank Nitti saw Capone was in 1946. The real Nitti killed himself in 1943.
Multiple scenes show Thompson submachine guns with drums inserted and the bolt forward. The actors then retract the bolt and start firing. This is not possible with a live Thompson. The bolt must be retracted to insert or remove the drum. When the bolt goes forward, it fires. Unless of course, the drum is empty. Thank you.
During the car chase, supposedly in the middle of Chicago, towering green mountains can be seen in the near distance at the ends of some streets, clearly indicating it's a studio set in California.