Continuity mistake: After Conan comes down from Valeria's funeral pyre, you see him throw down the torch he used to light it and everyone is in the dark. The film then cuts to a long distance shot from behind the rocks to show Valeria's pyre exploding again and you can see the torch in Conan's hand.
Factual error: When Conan cuts off Thulsa Doom's head, and throws it over the parapet, it hits the stairs below and rolls a little way down. Not only are there absolutely no marks left on the stairs (there should at least be a small amount of blood), it sounds like something plastic is rolling down there.
Revealing mistake: Several times throughout the movie you can see that some actors have contact lenses in their eyes. Obviously, there were no contact lenses during the Hyperborian Age. I don't mean the intended effects created with the contact lenses - for example, green eyes - I mean that you can see the actual lenses. You can see the circles outside the outer circumference of the iris, as is usual for soft lenses.
Revealing mistake: When Valeria cuts a soldier's throat, a protective pad over his belly protrudes under his shirt.
Continuity mistake: When Valeria fights a guard, his helmet falls off. A second later, she beheads him, and his helmet is back on.
Continuity mistake: While the dogs are chasing Conan, the sun changes between shots.
Continuity mistake: When Conan is captured and Thulsa scolds him for killing his snake, Conan's cloth is in rags by his elbow. Then in a close-up the cloth is not that torn apart and it's over his shoulder. Then a baddie rips it and back to a wide angle with the cloth in rags by Conan's elbow again. The close-up angle should've come first of all.
Continuity mistake: When Conan cuts off Thulsa Doom's head, Thulsa's arm swaps between bent or extended depending on the shots.
Continuity mistake: When Conan pushes the huge cauldron there's a man by the stairs who disappears in the immediate following angle.
Revealing mistake: After Conan pushes the heavy cauldron to spill the green liquid, the cauldron falls off and bounces like a football, revealing it's a prop.
Revealing mistake: When Conan kills the baddies in the orgy, he aims a blow at a man's neck, then punches a guy standing next to a column (and later stabs his stomach), but his punch misses by far.
Continuity mistake: When Valeria tries to grab the jewels Conan his fist is under his chin. A shot later it's on his cheek.
Continuity mistake: When Conan enters the witch's place and she throws something in the fireplace, he has his hand by his mouth. When the angle changes it's further away, next to his right shoulder.
Continuity mistake: When Conan is inside the cave and finds the sword, he turns his left hand around to grab the handle, but a shot later from a different angle his hand is in a totally different position, holding the sword from someplace else.
Revealing mistake: When the movie begins and Conan's father is talking, watch the pattern of the blue-screened clouds behind. The same clip is being repeated over and over again.
Continuity mistake: After Conan ends fighting in the ring he takes his hand to his neck, hair covering it. A shot later his hand is over his hair.
Revealing mistake: When Conan is fighting in the ring and smashes a guy's body against some [prop] rocks, the painting cracks and falls off.
Revealing mistake: When the dogs are eating Conan's dad, he is replaced by an obvious stuntman with a noticeable wig.
Continuity mistake: After Conan's dad dies two baddies walk towards the kid. The second one sheathes his sword twice.
Continuity mistake: When Conan is about to climb into the tunnel that leads to the snake pit, he has a dagger and on his back his sword, but when he is inside the snake pit, the sword is gone.
Answer: Like an Army Drill Sargent, he was berating them for not doing it better. Toughening them to become the best they could be.
Also, the other student laughs at Conan's punishment, so the teacher kicks him to teach him some humility.