In the "You're A Grand Old Flag" number, which supposedly takes place in the 1906 production of "George Washington Jr.," we see a group of Boy Scouts march onto the stage. The Scout Movement was founded in 1907 by Sir Robert Baden-Powell in England and wasn't founded in the United States until 1910.
The "You're A Grand Old Flag" number, supposedly takes place in the 1906 production of "George Washington Jr.," and uses multiple period flags to represent times before 1906. The Civil War flag, as an example, is correct for the time in question. However, in the final sequence characters carry, and an soft screen projection is made of, multiple 48 star flags. The 48 star flag was not introduced until 1912. In 1906, it should have been a 45 star flag. (Oklahoma was admitted to the Union in 1907, New Mexico and Arizona in 1912).
The writers stretch the bounds "poetic license" by trying to tie George M. Cohan's flop Popularity (1906) with the sinking of the Lusitania (1915) and the U.S. entry into World War I (1917) as all occurring at the same time.
In the "You're A Grand Old Flag" number, which supposedly takes place in the 1906 production of "George Washington Jr.," an African-American chorus pays tribute to a backdrop image of Abraham Lincoln, seated in a chair. The Lincoln image is taken from Daniel Chester French's sculpture for the Lincoln Memorial. This sculpture was not completed until the opening of the Lincoln Memorial in 1922.
Near the beginning of the movie, George M. Cohan is describing his birth (in 1878) to Franklin D. Roosevelt, and speaks of the flag "having fewer stars then, but people knew more were coming". The camera fades to a scene of an American flag waving on a flagpole, before panning down to people at a 4th of July parade waving hand held flags. The hand held flags appear to be the correct 38 star flags of 1878, but the flag on the pole is the 45 star flag introduced in 1896.
During the dock scene where Cohan is singing "Give My Regards to Broadway," the S.S. Hurrah steams away with a 48-star flag astern. The Broadway play from which the song came was produced during the time when the flag had only 45 stars.
The song "Off the Record", performed near the end when Cohan is portraying Franklin D. Roosevelt in the musical "I'd Rather Be Right", features some morale boosting anti-Nazi lyrics. However, "I'd Rather Be Right" played on Broadway in 1937, two years before World War II broke out, and four years before the U.S. entered it.
In the "You're A Grand Old Flag" number, which took place in the 1906 production of "George Washington Jr.," we see a group of Boy Scouts march onto the stage wearing the 1940 scout uniforms. In the beginning (after 1906) the uniform looked like an Army uniform.
In a montage following the news that the Lusitania has been sunk (a 1915 occurrence), a movie poster is seen for a film called EMPTY HOLSTERS. This was a Warner Brothers B-Western starring Dick Foran that was released in 1937.
Despite the film's storyline, and Cohan's own lifelong claim, that he was born on the 4th of July (and his having written the song "Yankee Doodle Dandy" containing that very line), George M. Cohan was in fact born on the 3rd of July (1878).
In the dressing room scene, just before Albee's visit, Jerry Cohan wraps a scarf around his neck while he's talking to George M. Cohan and leaves one end outside of his dressing gown. In the next shot, the scarf is tucked in.
In the shot preceding the "Yankee Doodle Dandy" number, a close up of the conductor's stand shows the conductor's music, which is only a "lead sheet" with the vocal line and lyrics only. This would never be the case, especially on Broadway. The conductor always had a "piano-conductor" part with vocal line and a 2 staff accompaniment of all the music in the show.