Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: The Muppet Show
The infamous Hugga Wugga skit, involving a Lyrical Dissonance from a stalked eyed creature singing "You are my Sunshine", the aforementioned creature who's violently abusive to anyone who doesn't sing the same lyrics as he does and the poor beaked sap that's abused by the former. It starts off creepy enough with the steam nose abuse, but takes a darker turn when the beaked monster starts singing the same song as the stalk eyed creature; and finally when the Hugga Wugga shoots off the stalk eyed creature's head he is legitimately confused and terrified when it continues to sing without a head
Two words : Inch Worm.
The whole Alice Cooper episode!
Vincent Price's version of "You've Got A Friend" starring the creepy muppet Uncle Deadly.
There was a skit featuring a gloomy guy in a room with skeleton singing about how happy he was. Mild nightmare fuel!
The Mummenschanz episode, especially the clay faces.
The Zero Mostel episode has a sketch where Mostel recites a creepy poem about literally banishing his fears (as in making various creepy-looking Muppets gathering around him vanish into thin air). It ends with him coming to the unsettling realization that he might be the embodiment of someone else's fears, and that they will make him disappear... which is exactly what happens. It gets worse when you realize that Mostel died before his episode aired.
"You're Always Welcome at Our House" (sings Marisa Berenson with a demented smile....)