1. When the spirit of the harvest moon basically looked like Satan.
Episode: "The House Of Discontent" (Season 2)
Why it was disturbing: Cartoons are animated for a reason. Not only is this clearly a human, but the human has holes for eyes and lips darker than his soul.
2. When Courage became a puppeteer for his owner's dead bodies.
3. When bad CGI had the ability to give you night terrors for weeks.
4. When the fetus blob from Hell reminded you of your imperfections.
5. When claymation was used for pure evil.
6. When Freaky Fred paid Courage a visit.
Episode: "Freaky Fred" (Season 2)
Why it was disturbing: A crooked grin and hair fetish makes Fred the most menacing barber ever. He also strictly spoke in rhymes and used the word "naughty" far too comfortably.
7. When an entrepreneurial Kat became a murderous sociopath.
Episode: "A Night At The Katz Motel" (Season 1)
Why it was disturbing: This episode solely determined whether or not you were a dog or cat person. His cool and collected demeanor only made him that much more sinister.
8. When even the tree had a human face.
Episode: "The Magic Tree of Nowhere" (Season 2)
Why it was disturbing: This half-smurf, half-tree humanoid is a good guy, but definitely something from your childhood nightmares... or possibly an adulthood acid trip. Human mouths in cartoons will never not be creepy.
9. When two zombies were about to devour Muriel... on camera.
Episode: "Everyone Wants To Direct" (Season 1)
Why it was disturbing: Eustace is about to unknowingly film his wife's murder while she's strapped down. The zombies are also former serial killers.
10. When Muriel became possessed by a mattress demon.
Episode: "The Demon in the Mattress" (Season 1)
Why it was disturbing: Demon possession, vomit, and decapitation. No explanation needed. Where was the Exorcist-inspired "Rugrats" episode though?
11. When a big fungal foot grew five personalities.
12. When you feared the bite of a Weremole.
Episode: "Night of the Weremoles" (Season 1)
Why it was disturbing: At least Werewolves look like oversized dogs. A bite from this fictional animal will just turn your elders into cannibalistic gargoyles.
13. When the mysterious mask was a dinner guest.
Episode: "The Mask" (Season 4)
Why it was disturbing: For one, a peeping tom is never cute. That mask may help her escape reality, but if that face was looking in through your window, you'd wanna escape too.