This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Human Centipede
The Human Centipede:
One might make an educated guess of what it is from the title, but it is so much worse. This clip alone can be emotionally scarring; do not view it under any circumstances if you are faint of heart or possess a weak stomach. Once viewed, it cannot be unseen.
For those not in the know it gives proof once and for all why you NEVER go ass to mouth.
Scariest Powerpoint Ever. You know, the one where he explains in graphic detail exactly how he's going to attach each 'segment' from one's mouth to another's anus, making a complete digestive tract?
Jenny sobbing brokenly for her mom and begging Lindsay for help as Heiter is forcefully sedating everyone before surgery. Poop in the mouth is one thing, but that kind of raw despair? That is disturbing.
Heiter himself is a Nightmare Fuel Station Attendant. A deranged Mad Doctor who loves to inflict pain to innocent people and then playing with them simply for giggles. The part when he explains the operation to his captives is quite unnerving. You can see pure evil madness in his face.
The Human Centipede 2:
Twelve people going ass to mouth instead of three people.