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)
Deliberate mistake: Captain Haddock finds out the SOS was a fake so he tells the engine room to increase speed. Just after they increase speed and Tintin says "The people financing the Peary", we can tell the boat isn't moving since the background is entirely still and not moving an inch. (00:14:45)
The Broken Ear: Part 2 - S2-E3
Deliberate mistake: When the villains' cuffs are unlocked, only one of the locks get unlocked, yet the cuffs come off both hands. (00:25:10)
Destination Moon: Part 2 - S3-E10
Deliberate mistake: The shot of the plane landing in Syldavia at the start of the episode, after the recap of part one, is merely the exact same shot of the plane landing in Syldavia at the start of the previous episode. Note the vehicles on the ground are completely identical and the plane, as well as its movements are also identical. (00:25:00)
Chosen answer: It is a pill that makes alcohol taste disgusting.