Prisoners of the Sun: Part 1 - S3-E5
Revealing mistake: When Tintin is swimming back to the boat after escaping the ship, Haddock is quickly rowing back and forth to reach him in time. As Haddock rows, the bars used to place the oars through can be seen moving back and forth, along with the movements of the oars, even though the bars are supposed to remain still during the oar movements. There are also frames from this shot where the bars vanish from the frame entirely as Haddock rows. (00:07:25)
Chosen answer: It is a pill that makes alcohol taste disgusting.