Revealing mistake: Conan and his party flee from the temple guards and escape into the crypt, closing the huge stone door behind them. A moment later, the camera shows the leader (Ferdinand Mayne) standing at the foot of the steps holding a torch. He chucks the torch out of sight to his right and begins walking towards the door. If you look closely you'll see the torch disappear nicely out of sight behind the wall as if he had "passed" it to someone standing there.
Continuity mistake: When Conan is fighting the half lizard/man monster, the first two times Conan smashes glass panes, you see large gashes appear across each of the monster's pectoral (chest) muscles. Later in the fight the wound on his right pectoral disappears and reappears numerous times.
Continuity mistake: Watch the shot where Bombaata is smashing a hole in the rock wall with his mace-axe weapon. From the outside it appears as though an entire section of the wall is giving way, then the camera cuts to Bonbaata on the inside and he is smashing at a rather small hole in the otherwise intact wall.
Continuity mistake: Two horsemen try to entrap Conan in a net during the opening fight sequence. He rolls under the net, grabs it and gives it a mighty heave, sending both horsemen sprawling, and then drops the net over Valeria's stone altar. A few shots later, the altar is bare and the net is nowhere to be seen.
Other mistake: When Conan is fighting the Ape-man and his friends are watching behind the mirror, the cold air on their breath is actually cigarette smoke. You can see it swirling every time they speak. In one shot you can actually see Malak's hand move behind the other actor and he's holding the cigarette. I think this is why there are audio problems here, they had to dub the voices in because the actors couldn't speak with the smoke in their mouths. DVD viewing.
Continuity mistake: Before entering the room with the wizard leader, Zula tosses the torch into a cubbyhole in the wall, but the cubbyhole isn't lit up by the torch as if it went out (or fell down a shaft). However, when escaping, Akiro grabs the lit torch out of the same cubbyhole, which is lit up by the torch (and he didn't use his magic to relight the torch).
Revealing mistake: During Conan's sword-fight with one of the Queen's guard (after the escape from the castle), you can tell the sparks were added in afterwards. At one point, Conan's back is to the camera and the guard attacks from Conan's front and a spark is seen in front of the back of Conan's head when his head should have blocked the view of the spark.
Revealing mistake: In both shots rowing toward the ice castle and back from it, the shot is obviously using some sort of rear projection, as the film running behind has marks such as hairs and scratches that have been amplified. The contrast between the main shot and the rear projection is different too.