In the first army scene where Glenn Miller conducts an army band on an airfield, a Boeing B-29 "Superfortress" bomber is seen in the background. At that moment the B-29 existed only in prototypes and was a well guarded secret, not for such wide display.
As the troops are marching on the parade field and Capt. Miller is conducting the band, the background shows different aircraft on static display. One aircraft is a HU-16 Albatross, used for Air and Sea rescue. This aircraft type did not fly until 1949 and was first used in Korea, 5 years after the movie date line takes place.
At a performance at a British airfield early in Miller's Army Air Corps, in the hangar behind a B-17, engine nacelles housing the B-29's Wright R-3350 Duplex Cyclone engines can be seen. No aircraft using these engines were in the Eastern Theater of Operations in WWII.
In the scene where James Stewart and June Allyson arrive in their car and are hounded by fans for autographs, one of the eager fans accidentally smashes into Allyson's head prompting her to hold her hand up to her face while she smiles.
In one scene when Glenn and Helen are discussing if he should quit his band job and continue his formal music studies, Glen begins the process of tying his necktie and is not wearing a jacket. The next time we see him - about 6 seconds later - his tie is done and he has on a suit jacket.
Towards the end of the movie, Helen, Si and Chummy are listening to the Glenn Miller Band's radio broadcast. Chummy MacGregor and Si Schribman are standing next to the radio in Helen's living room and Chummy is resting his hand on Si's right shoulder. In the next shot, Chummy's hand is no longer on Si's shoulder - it is hanging down between the two men.
When leaving at the airport, Glenn puts down his trombone on the left and his valise on the right to say goodbye to Helen and his children. After the goodbyes he picks up the trombone on his right and his valise on the left.
When Glenn (James Stewart) turns up late for their date Helen (June Allyson) is asleep. When she is woken by him and hops out of bed she has her shoes on. Later it looks like she goes back to the side of the bed to put on the shoes she is already wearing.
In perhaps the film's most notorious goof for the sake of dramatic license, Miller's famed swing instrumental of "Little Brown Jug" is depicted in the closing scene as a "special arrangement" Glenn created for a Christmas 1944 radio broadcast by Miller's AAF Band from Paris. In fact, it was one of the real Miller Band's first bona fide hits in 1939, arranged by the recently hired Bill Finegan, who became, along with arranger Jerry Gray, two of the key behind-the-scenes craftsmen that helped mold Miller's civilian band into the enduring commercial and artistic powerhouse it became.
When recording Tuxedo Junction for a film, the band are shown facing away from the screen. Firstly, the music is recorded before the dance is filmed, so the process is shown the wrong way round. Secondly, the band are facing away from the screen, so it would not have been possible for them to sync the music in any event.
When the air raid siren goes off during 'In The Mood' as a warning for the Doodlebug it is actually sounding the continuous 'Raiders Past' or 'All Clear' signal when the actual air raid warning was a wavering sound with the note changing in an approx 5 second pattern for 2 minutes.
The reed blend wasn't suddenly "discovered" in a single incident. Miller initially experimented with using a clarinet lead when he was arranging for Ray Noble's US band but it took almost three years for him to develop at the final voicing.
Pennsylvania 6-5000 is presented as the phone number of a house where Glenn is staying. It actually was (and still is) the number of the Hotel Pennsylvania in New York City, where the band performed at the Café Rouge.
Although the C-64 Norseman in which Miller disappears over the Channel is the correct aircraft type, it bears incorrect insignia, just a white star on a blue circular field without the blue and white side bars in use during the war.
A newspaper states that the Glenn Miller band will be performing at the "Glenn Island Casino". In the next scene they show the casino, but the sign reads "Glen Island Casino". The second "n" in "Glenn" is missing. In reality, Glen Island Casino is spelled with a single "n".