This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Boy Meets World
Season 6 had the appropriately-named "The Psychotic Episode". As if the main plot of Cory having nightmares in which he kills Shawn wasn't bad enough, the subplot with Eric's new roommate could be stretched out into a full horror movie! The man keeps the corpses of his mother, uncle, grandmother and pet parrot around and makes them talk! Plus, it's implied he killed all his other roommates.
And then we get to the ending: Feeny's actual advice to Cory is to let the original dream (of shoving Shawn down the elevator shaft) continue, as Cory kept waking up after Shawn went in. After shoving Shawn down the shaft, along with Jack, Rachel and Angela (Eric went down on his own), Lauren comes in, saying she represents everything Cory is giving up by marrying Topanga, before willingly jumping down the shaft. Then Topanga comes in, with a wedding dress, to mourn the deaths of their friends with Cory. In other words, Cory is so worried about his life changing once he got married, he kept dreaming of killing everyone he knows.
The flashback to Rachel accidentally destroying her stuffed rabbit in the garbage disposal is a bit unsettling.
The tarantula in the Honeymoon episode can be unsettling to arachnophobes.
The twist to "And Then There Was Shawn": the killer was Shawn himself... trying to get Cory and Topanga back together (since he feels them breaking up was his fault).
Pretty much every part of "And Then There Was Shawn" not played for comedy is terrifying. There's blood, explicit mention of death, impalements, and more. And the "strange, shrouded figure creeping behind us that none of us will see" is played for laughs, but the killer is terrifying with his skull mask. Good luck sleeping after watching it.
Special mention goes to The Stinger: Feeny has a dream in the middle of class of the students being polite and engaging. After waking up and seeing them rowdy and rambunctious, Feeny pulls out some scissors from his desk, and looks at it in contemplation.