When Boulton is preparing to fence with Major Andre, in the close-up his left arm is above his shoulder and arched, but in the two-shot it's below his shoulder and straight.
A British ship brings Andre and Bolton up the Hudson River for a meeting with Arnold. Andre leaves the ship for the meeting and eventually Bolton jumped ship and escaped the British. At this point the British and Americans exchange artillery fire. But there is no recoil with any of the artillery pieces, neither the British deck guns nor the American field pieces ever are thrown back from firing. This would be in violation of the laws of physics. Any piece that fires a projectile would be thrown back a couple of feet or so. Obviously, these pieces were loaded with powder but not waded, then lit to cause the powder to flash down and out of the barrel.
"Blow the Man Down" is heard on the soundtrack during a scene involving the man o' war. The sea shanty was composed anonymously in the 1860s, eighty-odd years after the incidents in this film.
The marching tune "The Girl I Left Behind Me" played by the band at the beginning of the film was written in 1791 more than ten years after the events of the film.
Major John Andre was only in his twenties, not the middle-aged man portrayed in the film.