Continuity mistake: In the scene when Conan's mother is killed, we see her using both hands to hold her sword toward Thulsa Doom and his men. The camera cuts to young Conan and he is holding her right hand. When the camera cuts back to her, she is holding the sword with both hands again.
Continuity mistake: Another one from the beginning of the film when Conan's mother is killed. Conan's mother is holding a sword, protecting her son. Thulsa Doom climbs down from his horse, takes off his helmet, and casts some kind of spell on her with his eyes. She lowers her sword and Doom slowly turns away, then spins quickly and cuts off her head. When the camera cuts to a longer shot, we see that Doom is about 15 feet away from her, much too far to reach her with a 3 foot sword.
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.
Continuity mistake: Just after his meeting with Moko (the "wizard"), Conan gets some flowers to take with him to the Mountain of Power. The color of the flowers change from scene to scene (they start out purple, but when Conan arrives at the base of the Mountain of Power, they're white/yellow) Note: this isn't a matter of Conan needing to get more flowers because of the great distance between the mounds (where the wizard lives) and the mountain of power. The scene transition from the Mounds to the Mountain of Power suggest a day's travel, and similarly Doom takes about a day to get to the Mounds from the Mountain of Power after Conan steals the princess. Flowers don't wilt and need replacing in a day's time.
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.
Continuity mistake: After Conan comes down from Valeria's funeral pyre, you see him throw down the torch he used to light it and everyone is in the dark. The film then cuts to a long distance shot from behind the rocks to show Valeria's pyre exploding again and you can see the torch in Conan's hand.
Factual error: When Conan cuts off Thulsa Doom's head, and throws it over the parapet, it hits the stairs below and rolls a little way down. Not only are there absolutely no marks left on the stairs (there should at least be a small amount of blood), it sounds like something plastic is rolling down there.
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.
Other mistake: When Conan gathers flowers to "take to the girl", the flowers that he is holding as he mounts his camel, are very long-stemmed, and dark purple in color. In the next scene he is riding the camel and the flowers he is holding are not only very changing color as another post indicated, but they are also very fake plastic / fabric flowers. In the same scene, Conan is then seen wearing a strong of flowers around his neck, those too are very fake flowers. (01:05:00)