In the scene where they are leaving the commissioner's office, they get in an elevator and push a button to go to the ground floor. All elevators at that time had operators who controlled elevator movement. Push buttons did not come in to use until the 1950's.
During the Duluth/Chicago game, the radio announcer declares that the quarterback has thrown a "Hail Mary" pass down field. The term "Hail Mary" pass wasn't coined until after the Dallas Cowboys beat the Minnesota Vikings in a December 28, 1975 NFC playoff game on a long last-second pass down field.
In the scene where they are being chased through the building after leaving the speakeasy, a glass and blue lit modern "exit" sign was clearly visible above the doorway. Also, modern fire sprinklers where seen in the ceiling of the same shot.
In 1925 when the film is supposed to take place, the FRC and the Office of Censorship were not even thought of yet. The Office of Censorship was established on December 19, 1941. The FRC (Federal Radio Commission) was established in 1926. So when the engineer said Sudsy and Lexi couldn't say things on the air, they could because there was no regulation.
The job placement worker asks Dodge if he learned a trade while he served in the war, then rhetorically asks how has he earned a living in the last 20 years without a marketable skill. The war ended 7 years before the story takes place.
During the bar fight between the football players and the Army men, the piano player is first shown playing while the fight is going on, a few shots later the piano player is shown and has a bottle now sitting on the keyboard (which he smashes over someone next to him).
Just before the police raid the speak easy. Lexie and Dodge are having a drink and Lexie's hand changes position between camera angles. Her hand is up then down and back and forth between camera shots.
In the opening scene when Carter Rutherford scores a touchdown for Princeton he runs before a packed house. Yet, when he's mobbed after scoring, empty terraces can be seen behind the first few rows of fans.
In the opening scene of the final game, the over the shoulder shot of the announcer and the engineer, the announcer is on the engineer's right. But when it cuts back to them with a medium shot, the announcer is on the engineer's left.
In the bar scene when Dodge recognizes his old friend Eddie, they talk for a while as the fight between the football players and service men take place around them. After a quick cutaway, they are shown still talking to each other, but now Eddie has a cut on his nose and lip.
There are several key historical errors. The NFL already had a league president in 1925 (Joe Carr), and he was not appointed by Congress. Moreover, he would not have had the power to deal with the media as he does in the film.
At one point in the movie someone says that Duluth has beaten Pottsville. However, the 1925 Pottsville Maroons only lost two games that season, to Frankford and Providence, on their way to beating Chicago in the League Championship. The League stripped the Maroons of their title after Pottsville defeated a Notre Dame team that included the Four Horsemen. At the time, it was widely recognized that Pottsville helped legitimize the fledgling league by not only beating the best team ever fielded up to that point, but through their use of a professional, team-oriented approach to their play, instead of the barnstorming methods employed up to that point.
Lexie reports that Duluth has beaten a woeful Pottsville team. However, the 1925 Pottsville Maroons only lost two games that season, to Frankford and Providence, on their way to beating Chicago in the League Championship. Nonetheless, the movie is a fictional story, as this movie is a fiction and need not adhere to historical match records.
The story takes place in 1925. America entered World War I in April 1917 when Congress, at the request of Woodrow Wilson, declared war against Germany. The war ended with the Armistice in November 1918. Even if Carter had been a freshman when he joined the Army, he would have finished his studies by 1921 or 1922. In short, he wouldn't still be at Princeton in 1925.
At the end of the movie, the series of newspaper headlines state some related information, but all of the articles contain generic paragraphs, none of which have anything to do with the headline or the plot. Other headlines on the pages appear multiple times, such as "KING IN NEST OF SHRINES", taken from the New York Times' 1923 article on the opening of Tutankhamen's tomb.
The goof item below may give away important plot points.
After the 'Sgt. York' play, Jimmy (George Clooney) is doused with water, revealing his Duluth uniform. When he walks away from the mob, the front of his jersey is still covered with mud. In his ensuing conversation with Carter (John Krasinski), the partially-shown jersey is again rinsed off enough to reveal its color.