Just after Conan and Malak are surrounded by Taramis' guards at the start of the film, Malak asks, "Why are they trying to kill us?" As Conan answers, the same line of dialogue can be faintly heard playing again.
When Conan is fighting Man Ape and discovers that breaking the glass panes hurts him, it cuts to a shot of his friends behind one of the panes, and Malak says, "That's exactly what I would have done, exactly." His lips don't match the words at all.
Just after Akiro reads the legend on the wall of the crypt regarding the woman-child with a certain mark, we are treated to a view of the mark that makes Jehnna so special on the swell of her left breast (a pink star shaped mark). However, earlier in the movie, when Jehnna first awakens in the castle of Toth-Amon as well as the scene a few minutes after this when she lifts the jewel/key from its place, this same area of her chest is perfectly visible but the mark is not there.
When Zula is fighting Bombaata, she leaps from the saddle and throws herself on him, knocking his head-circlet off from the impact of collision. In the very next shot, Bombaata's circlet is back on his head as he struggles with Zula.
During the opening sequences between the sword fight with Togra and Conan. You can see Togra wearing a long cape in the long shot. When they do a closeup of Togra drawing his sword, he's not wearing the cape anymore, but when they charge at each other on horseback Togra is wearing his cape again. When the swords clash in the closeup, the cape is once again missing for the rest of the fight.
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.
The four bloody scratches on Conan's arms during the Man Ape fight scene, the blood slightly ran down Conan's arms, but minutes later just as the crystal palace begins to crumble, those bloody scratches are on his arms neatly with no blood smearing.
At the beginning of the fight against the Man Ape, the Man Ape swipes his claw, striking the left side of Conan's head which knocks him down. Conan holds that spot in pain. A moment later, while Man Ape has Conan in a choke hold, he also scratches his face in the same place where was just claw-swiped. Those claw and scratch marks on his face and left side of his head are visible.
During the Man Ape fight, when the Man Ape has Conan in a bear hug, you can see that Conan's back is perfectly clean, then a moment later when Conan is lifted and carried on the Man Ape's shoulder, you can see a ton of blood on his back from rubbing off the Man Ape's chest. Another moment later, while Conan is breaking the mirrors, all the blood is gone.
The goof items below may give away important plot points.
When Conan is rowing the rudder of the boat escaping from the wizard's collapsing castle, in the frontal shot from the back of the boat, the skull is on top of the rudder and a spike points down, in the rear shot of the party the skull is on bottom and the spike points up.
During the battle with Dagoth, bolts of lightning destroy several columns and arches in the throne room. However, when Jehnna is bestowing Conan's companions with rewards, there is no sign of damage to the room.
As the ice palace is collapsing and Conan is starting to row the boat with his companions with him, he is wearing just a loincloth, but while still rowing the boat in the middle of the lake, he is fully clothed.