The giant spider in "Tintin The Shooting Star". First Tintin has a nightmare where Philippus the prophet shows him a a full size poster depicting a huge black spider, with the caption: "real life measurements". Later Tintin visits the planetarium and views through the telescope when he sees another gigantic spider. It turns out it is just a small one on top of the lens. But later, as Tintin travels to the comet which crashed into the sea it turns out there is a giant spider after all! Grown to the size of a large dog due to the comet's radiation!
The dream and nightmare sequences in "Tintin" are notoriously surreal and downright creepy:
In "Tintin Cigars of the Pharaoh" Tintin is locked inside an Egyptian tomb and put to sleep with sleeping gas. He then dreams several strange images combining recent people he met and Egyptian artwork.
In "Tintin The Shooting Star" Tintin dreams he is visited by Philippulus the prophet who then shows him a picture of a gigantic spider, claiming it is life size!
In "Tintin The Seven Crystal Balls" Tintin and his companions all have the same nightmare: that they are visited by the Inca mummy Rascar Capac who enters their bedroom by night and then throws a poisonous crystal ball on the floor. It's probably the scariest moment out of all the books. The Slasher Smile on the mummy's face makes it even worse.
This is even more terrifying in the animated series.
Also from Tintin The Seven Crystal Balls, the seven Egyptologists that have been rendered comatose simultaneously regain consciousness at the same time every day and scream in terror before returning to their comatose state. It's never said what is making them scream like that except for a reference in Prisoners of the Sun about them being tormented. Of course it is much worse in the animated series; in the comics it was just one panel of all seven of them writhing about, in the series we're treated to the sight of their weirdly blueish-grey skin and close ups of them screaming which is itself horrific to listen to.
It's much more terrifying because we don't get to see their hallucinations....a good theory is Rascar Capac and other Incan Mummies coming to life, appearing and trying to kill them in various horrible ways, with those massive slasher smiles on their faces. And they have to endure that every single day. Brrrr.
In "Tintin Tintin In Tibet" Haddock dreams he meets Professor Calculus, who claims he has lost his umbrella. Haddock then tells him he's got a lot of umbrellas with him, but has no idea where they came from. Calculus is angried by his answer and tells him: "You lie! It's red pepper." Then Haddock suddenly wears Calculus' clothes, while Calculus wears those of Haddock. Now grown to enormous size Calculus hits Haddock on the head wih an umbrella, claiming it's "Checkmate!"
Here too, the effect is amplified by the animated version.
In "Tintin The Castafiore Emerald" Captain Haddock dreams he is listening to an opera singing parrot while he is seated completely nude in an audience consisting of nothing but parrots.
In Tintin Prisoners Of The Sun, Tintin dreams that Calculus is admiring an "Inca Tree" whose flowers are skulls while a real Inca menaces him with a spear, next Tintin asks the Inca, who now has Haddock's face if he has a licence for the rifle over his shoulder, upon which the Inca turns into a mysterious Indian who has been following the heroes and blasts Tintin with fire for blasphemy. Tintin then wakes up with hot sunlight on his face.
The Rajaija poison used by the opium cartel in "The Pharaohs Cigars/The Blue Lotus". A single blowdart with it will drive any person insane, and no one is safe.
There's the very disturbing end of Tintin Flight 714 in which one character is revealed to be a human in contact with aliens, possibly a spy, and he uses Mind Control to save the heroes and forces the criminals into his ship. We don't find out what happened to them... but that just makes it worse.
Word Of God states that they eventually turned up alive and amnesiac on a deserted island.
Throughout "Tintin Tintin In Tibet", the yeti is portrayed with a great deal of sympathy (and as a the occasional butt of a joke), but when he catches Tintin in his cave, trying to make off with Chang... brrrrrr.
Frank Wolff's suicide in "Tintin Explorers On The Moon". He leaves the rocket. Even though it happens off screen and is explained in a suicide note: it's still very disturbing.
Made worse with the fact that the reader gets to imagine his horrible death and the fact that his body could be floating around in space forever...!
Mr. Wang's Son's insanity, and him going to DECAPITATE HIS OWN PARENTS AND FAMILY near the climax of "Tintin The Blue Lotus".
The mummified corpses of the doomed Egyptologists and archaeologists in "Cigars Of The Pharaoh". Remember, these were archaeologists in a coma who were mummified alive. The blank stares make it worse too. It's like they're watching you, and they know you're next...
What's worse? They're on the cover of the book itself.
Try to think of their last moments of hallucinating and mummification. And remember, these are supposed to be corpses, murdered by the drug smugglers.
Then there's the scene where Tintin finds three new sarcophagi, being prepared for himself, Snowy and the newest archaeologist.
In "Tintin Tintin In America", Tintin and Snowy falling onto the conveyor belt in the factory and about to be made into corned beef and eaten. Thank god for the striking workers, who stopped the conveyor belt out of anger at their wage cut. And in the same album, the cow being turned into corned beef alive, and the "missing pet" notices outside the factory, when you realize that the pets were also made into beef....you will never look at corned beef the same again after that.