Continuity mistake: The scene where they are looking for Ryan's dog tag. Doc comments they are laughing in front of all the guys walking by. One of the soldiers looks like Sal Mineo. Frustrated, Tom Hanks walks into the line of soldiers and civilians and starts asking if anybody has seen or knows James Ryan. The same Sal Mineo lookalike walks by him again. (01:15:10)
Plot hole: Alain joins the Legion, goes through extensive training and eventually gets his photograph taken. That's how Galgani recognises him and sends two men to kill him. They arrive with a platoon at the fort just after Alain, Mackintosh and Luther. The photograph would have taken a while to get back to France and then the two men would have to go through the same training. So how do they both end up in the same fortress after they joined the legion long after Alain?
Factual error: When Pellew engages the first French warship, he gives the order to his helmsman, "Hard to Larboard". In the 18th century, helm orders were given in relation to the position of the tiller, rather than the rudder. A helmsman would push a tiller in the opposite direction he wanted the vessel to turn. This practice continued even when a ship was steered by a wheel, so the man at the wheel would have turned the wheel to starboard..except that he didn't.
Factual error: When the Spanish sink the topsail schooner bringing supplies to Gibraltar, Captain Foster swims to a floating fighting top. The schooner actually had nothing more elaborate than a crosstrees on both masts.