Factual error: The Vikings raided England in the 9th and 10th centuries. The first castles were not built until post 1066. The castle in the film has rounded towers, crenellations and machicolations, which would date it at approximately 1300. That would tend to suggest that these Vikings were 400 or 500 years old, or that they had access to a time machine.
Continuity mistake: When Kirk Douglas smashes through the chapel window near the end of the film, you can clearly see that the window isn't stained glass - it's colored paper. Or perhaps papier-mache of some sort.
Visible crew/equipment: When Kirk Douglas returns in the viking ship to his home fjord, many vehicles can be seen on the road through the trees on the other side of the water. Vauxhall Vikings perhaps?
Revealing mistake: When Morgana's ship is being boarded, one of the Vikings features an arrow stuck in his body even as he boards. It is first (but not quite) kept out of sight by his shield, but when he enters the ship, he turns the shield away, and the arrow is suddenly visible. Somewhat hard to see by casual watching, so look carefully.
Revealing mistake: During the scene where the Vikings are attacking the castle, a soldier gets an arrow straight across the neck side on. Given that the Vikings were 40 feet below and head on to the ramparts, how on earth was that shot made?
Continuity mistake: When Eric and the others are making their escape in the boat, the ladies are in front at first. Later on they are behind the men as they all row the boat.
Factual error: At the end, when Kirk Douglas throws his head back and screams: "ODIN!", you can see several mercury amalgam fillings in his teeth - not available under most 900AD health plans.
Continuity mistake: When Kirk Douglas climbs the axes thrust against the drawbridge, he is unarmed. After he lowers the bridge, his sword is strapped across his back, only to reappear in the following shots on his belt.
Visible crew/equipment: While Raynar and Einar are arguing about the wench, a crewman surreptitiously sneaks in a mic at the bottom left of the screen. (00:47:54)
Revealing mistake: Tony Curtis has his hand cut off in the film, and still his arm hasn't lost any of his blood or length.
Revealing mistake: When Kirk Douglas climbs the rope to the tower's window, you can see the flimsy rope. Supporting his weight, the rope should have been stretched tight. Right before he crashes through the window, you can make out the equipment supporting the stuntman. (01:40:20)
