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.
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.