This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Avenging Disco Father
The otherwise hilarious Blaxploitation film has an incredibly disturbing scene in which a woman high on drugs cooks her baby and serves it to her family. The whole movie has a pretty strong Drugs Are Bad message, but for the most part it's sort of tongue-in-cheek. Until then.