Nightmare Fuel is a trope where something unexpected happens on-screen that scares the living shit out of you to the point that it will probably haunt you for the rest of your life.
Nightmare fuel is one of those things that none of us think about until we are unexpectedly subjected to it. It crops up in the weirdest places, like real life and kids movies. It works best when you don't see it coming, like the Large Marge scene in Pee Wee's Big Adventure. It's a wacky goofball comedy for kids with a OHJESUSSHITWHATTHEFUCK?!?! Moment that comes out of nowhere and ruined your childhood.
Seriously, Burton. What the fuck?