The Secret of the Unicorn: Part 1 - S1-E3
Deliberate mistake: After Francis Haddock kills Red Rackham, he lights a fuse to the explosives. The fuse that was lit is only a foot long, yet he is somehow able to climb out the ship without anyone noticing, get into a boat and swim all the way to land before it explodes. At a foot long it would have exploded in 10 - 15 seconds, all of that would taken at least 10 minutes. (00:18:45)
Red Rackham's Treasure - S1-E5
Deliberate mistake: Haddock draws an X on the shark image of Calculus' box with a pencil. This was done as a visual gag, though the X mark is far too thick for the pencil that Haddock was using. (00:07:00)
The Black Island: Part 1 - S1-E10
Deliberate mistake: Tintin gets his leg stuck in a man trap. All scenes after this show no ripping in his socks. (00:14:40)
Chosen answer: It is a pill that makes alcohol taste disgusting.