Revealing mistake: Before the final battle, just after Katsumoto says that the Imperial army will not surrender, there is a cut, appearing when the Japanese army are ready to attack. You can see, at the right side of the screen, a transparent man walking in front of the troops. Actually you can only see a pair of legs walking without any body. (01:50:15)
Continuity mistake: When Algren is trying to prove to the Colonel that the soldiers are not ready to fight, by having the man try to shoot him, the first barrel band on the soldier's musket (the one closest to the muzzle) is detached and dangling just behind the front sight. It stays in this position for several shots and then magically resets itself after the weapon is fired.
Continuity mistake: Just before the final battle, a clip of the now well-trained Japanese army standing at "port arms" (holding their rifles in front of them, with their left hand on the upper stock and their right hand behind the trigger guard; the rifle is pointed up and to their left). This is a standard out of any military manual of arms. The scene flashes to several other snapshots of the other parties preparing for the big battle. When the camera shows another snapshot of the Japanese army, the film is in reverse. The soldiers are still at "port arms," but their rifles are pointed up and to their right.
Continuity mistake: When Algren is packing to leave, Colonel Bagley picks up the note that the young boy had written for Algren. In the close up, you can see the paper is light blue at the top and a light pink at the bottom. When Algren takes it from him and smooths it out, it is suddenly all light blue.David Mercier
Continuity mistake: When Algren teaches a soldier to shoot, the soldier has a chin strap below his lip. Yet, in the next shot there is no strap. (00:19:35)
Continuity mistake: When Algren is handing the 'sword' back to the little kid, his hands change position around the sword everytime the angle switches: first it's between his fingers, then fully grasped, then between his fingers again.