Revealing mistake: In the scene after the battle of Agincort, where Henry and his retainers are walking meditatively among the heaps of the fallen, if you look carefully, you will see one of the fallen with his head raised watching the procession. He then realizes that he is on camera and puts his head back down. It's near the edge of the screen so may occasionally be cropped.
Revealing mistake: During the transport of the POWs by train (which starts about 26 mins into the film) the number on the side of the engine is visible as 719. About 3 mins (filmwise) into the journey it appears in mirror image. It appears they filmed the train moving from right to left, but throughout the film the train moves from left to right, so the film must have been reversed. It happens twice, the second time 14 min 24 sec after the first. (00:29:34)