This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Stanisław Lem
"The Futurological Congress" has many such moments, but the second half or so takes the cake, growing scarier and scarier as it gets to the ending. Basically, the utopian future in which most of the story was set is shown to be a hallucinogen-powered Lotus Eater Machine designed to hide the bleak, dystopian reality from the masses, so as to avoid a panic and desperate fighting over the scarce resources. The members of the scientist council behind this have access to a counter-drug, but one member ends up with stronger and stronger versions that paint bleaker and bleaker pictures of the truth of humanity. The ending is sort of optimistic, as it all turned out to be a hallucination and the protagonist wakes up back in the present, but that scarcely dulls the sheer horror.
The Investigation, with its almost believable version of walking corpses.