Revealing mistake: When Conan cuts his way through the orgy we first see him cut a man in his stomach region - the bag of blood hidden under his clothes. Conan then cuts another man in the neck, but blood doesn't begin to flow until that man reaches up to his neck, making it rather obvious that he has his bag of blood in his hand.
Revealing mistake: Before the crucifixion scene of Conan, Thulsa tells one of the women on the cliff, "Come to me child," so they would jump off. If you watch closely you'll see, right before she hits, the pad she lands on before the angle changes. In the next shot, they then show her lying through broken wood flooring.
Revealing mistake: Several times throughout the movie you can see that some actors have contact lenses in their eyes. Obviously, there were no contact lenses during the Hyperborian Age. I don't mean the intended effects created with the contact lenses - for example, green eyes - I mean that you can see the actual lenses. You can see the circles outside the outer circumference of the iris, as is usual for soft lenses.