The Crab with the Golden Claws: Part 1 - S1-E1
Revealing mistake: After the crew member with the club has knocked out Tintin, he begins to chase after Snowy. Snowy begins running and stops when he runs into one of the crates. On the opposite side of the crate, at the very bottom, patches of color are visibly missing from the crate.
Chosen answer: If Hector was seriously intent on removing King Ottokar's scepter from the glass cage holding it and launching it through the window with the camera, then Hector would have been forced to knock out the two guards so they would not have interfered with Hector's plan as soon as he removed the scepter from the glass. As for Hector, himself, was explained when Tintin was explaining how Hector stole the scepter. Tintin placed a twig inside the camera and it launched out of the camera. Hector would have placed the scepter inside of the camera, and would have been knocked unconscious due to the spring effect created when the scepter had been launched.
Casual Person