(at around 45 mins) During the chase scene; just after Tamara disconnects the coach that she was riding in from the horses that she is now riding, you see the coach tip forward, roll and shatter apart. In the next shot the coach is whole again and lands on the front and shatters for a second time.
(at around 31 mins) When Ela Shan the escaped prisoner first appears at bar sits down to hide he puts his right hand up to hide his face and no manacle is present; subsequently, when Conan goes to arm wrestle him with his right arm he has manacle on his wrist.
(at around 1h 12 mins) During the fight on the ship, Conan is thrown back onto a raised platform in the cargo hold. When his legs fly up you can see the rubber soles of the athletic shoes built into his leggings.
(at around 58 mins) Shortly after Conan kidnaps Tamara, he puts her on the back of his horse with her hands tied behind her back. Riding like this would be impractical to the point of impossible. Assuming she didn't immediately fall off and break her neck, she'd be in unbearable agony within minutes of bouncing around in that position.
(at around 1h 17 mins) When Tamara is ambushed by Zim's men in the forest, she does not notice the first rider until he is only a few yards from her, despite having a very clear line of sight for a few hundred yards in every direction, and despite there being no noise louder than birdsong to mask the thundering hooves of a galloping horse. Similarly, the foot-soldiers were quite clearly not lying in wait - the trees in the forest are too skinny for them to have hidden behind and there is no other cover around - yet they are upon Tamara within a couple of seconds of the horseman attacking.
(at around 1h 17 mins) With no apparent way of knowing where Conan's ship has laid anchor, and with no way of knowing that Tamara would be on land with Conan and not still aboard the ship, Zim's soldiers nevertheless are able to set up a perfect ambush to try and capture Tamara after she leaves Conan's chamber and wanders through the forest.
(at around 1h 35 mins) In the climax when Khalar Zym and Conan are fighting, Conan hits Zym on his face with the back of his sword and he bleeds. Then Zym is shown with no blood on his face, but in the subsequent scene the blood is back on his face.
The goof item below may give away important plot points.
: (at around 1h 29 mins) When Khalar Zym begins the ceremony to make him a god, he cuts Tamara's chest along her sternum to get her blood. But for the rest of the movie, every time we see the cut on her chest, it is running more side to side instead of up and down.
(at around 9 mins) When Conan is attacked by enemy warriors, while trying to earn the right to fight with the warriors, he kills four, but when he presents their heads to his father on returning to the village, there are only three heads. This could, however, be attributed to Conan's running a warrior's head into a log and killing him, thus rendering his head not suitable as a trophy.