Factual error: When the riders are watching the wagon train, in the background you can see a communication tower with conical speakers.
Revealing mistake: When the American pilots flying P-47 Thunderbolts are attacked by German fighters, the German planes are really American P-51 Mustangs painted with German markings.
Revealing mistake: When Howard and Curtin chase Dobbs, they pass a dead donkey which is obviously breathing, and the rope used to tie its legs together is visible too.
Plot hole: When Ralls runs the Witch onto the reef, he is at the helm. As she strikes bow on, the wheel spins madly for no reason, as the tiller ropes would not have been affected by the collision.