Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: American Horror Story: Murder House
In the pilot we see Violet leading Leah into the basement with the promise of Cocaine. What happens instead is that she gets attacked by Tate and the Infantata.
Viv being put into a mental institution through Hayden's manipulations and Violet's lying and no one believing her.
Violet finding her body. There are flies in her mouth and her eyes are rolled back into her head.
Harvey's dead, horribly-burned wife and children acting as though they were still alive.
The school shooting sequence. Tate meticulously shoots people one by one as they plead for him not to. And then you remember we saw what became of the the Dead Breakfast Club just one episode prior. The nerd's injuries were so bad he couldn't even speak. Damn.