(at around 5 mins) 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.
(at around 43 mins) 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.
(at around 1h 8 mins) 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 (at around 45 mins), 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.
(at around 26 mins) 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.
(at around 49 mins) 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.
(at around 17 mins) Shortly before Akiro is rescued from the cannibals and joins the party there is a scene where Akiro can be seen on the last horse riding through the forest despite not having joined the party yet.
(at around 7 mins) After the initial fight, when Queen Taramis is approaching Conan, he stands up and takes a fighting stance. While standing, one of the nets gets caught on his scabbard (or knife hilt). Later the net disappears while he is still in the fighting stance and did not have time to remove it.
(at around 54 mins) After Princess Jehnna rubs the healing ointment on Conan's bicep, it runs down in several places. After Conan wakes up, we see his arm and the ointment goes across his bicep in a straight line.
(at around 5 mins) 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.
(at around 46 mins) 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 around 1h 20 mins) During the start of the fight against the temple guards, Conan throws his dagger into one of the guards. Later on when they reunite with their horses, the dagger is back in its place.
(at around 40 mins) 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.
(at around 42 mins) 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.
(at around 47 mins) During the escape from Thoth-Amon's collapsing tower, the silhouette of a crew member or a mechanism is visible inside one of the rocks (the largest rock just left of center at the top of the screen.) Just before the scene cuts to the next one, a shadow of a crew member is visible at the extreme left of the screen.
(at around 32 mins) When the heroes are 'rowing' toward the castle, Conan is 'rowing' with the rudder, which is clearly not designed for this purpose, and the oars are nowhere in sight. It could be said that rowing with this type of rudder is possible, using a vertical rhythm that ran in parallel with the skiff (boat), however Conan was rowing in perpendicular - from left to right.
(at around 50 mins) In Conan's first fight with the barbarian that steals the princess off the horse, he and Conan engage in a horseback sword fight. You can see both when Conan holds his sword up waiting for the enemy to strike his "block" and you can see him eying up the enemy sword as the target, rather than the enemy himself. Once near the end of the horseback segment, Conan actually hits his opponents hilt/fingers, rather than the blade (a filming mistake) but the fight continues as though it had been blocked by the enemy.
The goof items below may give away important plot points.
(at around 1h 30 mins) 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.
(at around 48 mins) 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.
(at around 48 mins) 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.