Factual error: The movie takes place during the year 1919. Yet before one of the World Series games the stadium announcer requests that everyone stand up to sing "the national anthem". The US did not have a national anthem until the 1930's when Frankin D. Rosevelt signed into law the Star Spangled Banner as the nation anthem.
Factual error: At the beginning of the film, in the shots of the players in action being voiced over by Comiskey, Cicotte is marking the ball with his fingernail. You can see the modern logo of the sporting goods company "Rawlings" on the ball. It can be seen even more later in the film when Comiskey's assistant is turning a team-autographed ball over in his hands. Rawlings did not start making official major league baseballs until 1976, and even if they had made them in 1919, they very likely did not have the exact same logo. (00:04:30 - 01:28:55)