Hey Arnold!/Nightmare Fuel


 * "Part-time Friends". Arnold and Gerald work together at a flower shop with Gerald being the boss since the owner injured her leg and appointed him as the temporary manager. After Arnold gets into a fight with Gerald and says that he doesn't want to be friends with him, Arnold's Grandpa tells him about a similar incident that happened to Grandpa himself when he was Arnold's age. Arnold then begins to have a nightmare dream where he and Gerald are old, still former friends, and can't remember what they were fighting about. The dream takes a turn for the disturbing when it cuts to the old, withered, corpse of Arnold's Grandpa who's still alive where he says something along the lines of "Well, what did I tell ya, Short Man?" while some ominous music plays. He then proceeds to laugh in a really, creepy way, only for his jaw to crack and fall off his skull.
 * There was one episode where Arnold and the gang are telling scary stories. Arnold's tale is called "The Headless Cabbie" which involves a cabbie driver picking up a woman who wants to go to the park because she's lost her little dog (she even gives the cabbie a scarf since it's late at night and he won't be cold). As the cabbie drives through the park, he hears barking, and the lady says that's her dog. The cabbie keeps driving, the barking continues, he doesn't see the little dog, but the lady keeps imploring him to go faster and faster, sounding and becoming more demonic. The story reaches its climax when the scarf gets caught on a tree branch, and since the cabbie is going so fast, the scarf winds tighter and tighter... until it finally cuts his head clean off. Not surprisingly, Harold (one of the guys that was hanging out with Arnold and co. at the time) is traumatized by the story.
 * The ending of that episode is pretty scary through implications.
 * The town and the school flooding in "The Flood". Worse still, all the kids are trapped in the school.