After watching "Vampire Bats and Scaredy Cats" (The Scooby-Doo Show), this troper, as a young child, had to sleep with a full head of garlic on her nightstand. For a month. Literally had nightmares about vampires coming to kill her.
One classic ad on Cartoon Network featured characters from Scooby-Doo: Shaggy showed up in a spooky castle calling out, "Scooby-Doo! Where are you?" This cut to Scooby on a tropical beach, saying, "Right here, Raggy! On Rartoon Network!" What was scary about it was not only the notion that Scooby would desert his friends in the middle of a case, but also the fact that Shaggy was barely animated at all, paralyzed and semi-slouched in seeming defeat as he called for help. It was haunting.