The Crab with the Golden Claws: Part 1 - S1-E1
Other mistake: Whilst examining the wrapper with "Karaboudjan" written on it, the "K" is a centimeter away from the letter "A" and all the other letters are about a fifth of a centimeter away from the letters. Tintin then places a magnifying glass above the word "Karaboudjan." After doing this, the "K" that was a centimeter away from the "A" is now a fifth of a centimeter away from the "A", but none of the other letters seem to have changed distances. This all occurs within the same shot. (00:05:15)