Cigars of the Pharaoh: Part 2 - S1-E7
Continuity mistake: As the doctor and Tintin walk through the mental hospital, they stop at the first door on the corridor. After the doctor walks back the way he came, he hasn't walked past the door he locked Tintin in yet, indicating that it is now the second door on the corridor he locked Tintin in. (00:33:30)