In the final fight scene the polearm that Des Roches is using breaks in two. The camera then cuts to a close up of Des Roches pressing in on Jamie. But Des Roches' polearm is no longer broken. Instead it is back to its original full length.
Early in the film, the trumpeter who sounds the alarm is shot by a crossbow. But the missile that hits him is an arrow and NOT a bolt. Arrows are shot by a bow and are long. Bolts are shot by crossbows and are short (half as long as an arrow).
Throughout this film, archers shoot soldiers in chain mail with arrows that have broadheads, But an arrow with a broadhead cannot penetrate chain mail. Arrows with broadheads were used in hunting game or against men with NO armor. Instead, an archer who intended to shoot a soldier in armor would use an arrow with a long narrow spike like point. These arrows were designed to go through the rings that made chain mail. By going through the gaps that comprised chain mail, the arrow head could then easily pierce the padding worn under chain mail and penetrate (wounding or killing) the soldier.
When the woodsmen ambush the soldiers, some of them use crossbows. The English did not use crossbows, not even after the Norman invasion. Instead, they used the long bow.
Throughout the film there are scenes featuring an exterior shot of the front of Des Roches' castle. Every time we see this, there is a horse and rider standing near the main entrance. This horse never moves and is always in exactly the same position, revealing it to be part of the painted backdrop.
When the men are trapped in the house, one is shot by an arrow. The prop that sticks out of his chest is short, which would mean that most of the arrow has passed through his body. As arrows are long, the head of the arrow would have been sticking out of his back. But when he is rolled over, there is no arrow protruding from his back.
When Jamie gets out of the boat and is set upon by soldiers, he uses a rapier. The Soldiers, however, used broadswords. As broadswords are heavier than rapiers, the soldiers would have easily disarmed Jamie by battering his rapier out of his hand or broken his rapier. Such a sword fight would have been short with Jamie the loser, unless, of course, the actors were merely tapping their swords together in mock combat.
A knight in chain mail (including chain mail pants) climbs a tree for a better look. But chain mail is heavy and no one dressed as such would be able to climb a tree, particularly as nimbly as this knight did, unless the mail was really cloth clothing painted gray to give it the appearance of mail.
James escapes the soldiers by using a staff to bridge two towers. He pulls up the staff to prevent their pursuit. As one of the soldiers comes to the battlement, he bumps the battlement and it moves ever so slightly. But as battlements were made of stone, that could not happen, UNLESS, of course, they were made of cardboard.
During the final duel between Desroches and Jamie, one of them smashes his spear into the stone wall, which, being actually made of cork, chips away.
When Deering dresses to pose as a lady, she is clearly wearing something akin to that of panties. But women in the Middle Ages did NOT wear underwear (women would not start to wear underpants until the end of the 18th Century).
When Jamie gets out of the boat and is set upon by soldiers, he used a narrow blade sword. This sword is a rapier and emerged in the middle of the 15th Century in Spain. As the time setting of this film is the early 13th Century, this weapon is out of place by over two hundred years. Instead Jamie should have used a broadsword, like the soldiers who accosted him did.