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?