The Calculus Affair: Part 1 - S1-E12
Continuity mistake: When Tintin and Haddock arrive at the hotel, they start talking to the guy at the counter. They ask about Calculus and the guy turns his head to the keys. The next shot shows him looking at the key to 122. If you look in the previous shot, you see the key hook for room 122 are in the far background. (00:12:35)