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.