The fly screaming "Help me, help me!" while he ate it in the toy village.
Made even worse if you recall that Geena Davis starred in the remake of The Fly two years earlier.
When the Maitlands are being exorcised, and begin to rapidly age before everyone's eyes.
The scene where Barbara tries to scare Delia by hanging herself in a closet and then ripping off her own face.
Fortunately for Delia and Otho, they can't see her. They were more terrified at how small the closet was.
I'm surprised no mention was made of Barbara and Adam's transformations where Barbara stretched her face with her eyeballs in her mouth and Adam made his face beak-shaped with eyeballs on his fingers. Look◊, if you◊ dare◊.
Shrunken[1]guy◊, guy RUN OVER BY A CAR AND REDUCED TO A REALISTIC PANCAKE, and a guy who SET HIMSELF ON FIRE FROM SMOKING AND WAS ALL CHARRED UP AND SHRUNKEN...
Betelgeuse himself qualifies as Nightmare Fuel...and not just when he's in his giant-snake form. He's pretty much Chaotic Neutral incarnate, can change form at will into pretty much anything, and he's not above killing people.