King Ottokar's Sceptre: Part 2 - S2-E5
Other mistake: A museum curator notices that the leg of a dinosaur bone head been stolen after Snowy removed it. During this shot, the dinosaur's foot hasn't moved an inch. How can this be possible? Snowy managed to remove a dino leg with his teeth, so must have dislodged the rest of the bone while doing so. (00:26:55)