Link to the story: Friendship is Optimal. Though I wouldn’t call the story as a whole a horror story; rather, it has some fridge horror. And it is particularly horrifying to those interested in the singularity, rather than to rationalists in general.
Doesn't the recursive fic Caelum Est Conterrens explore the horror aspects a bit more?
This is a kind of "X files" thread.
Post experiences which spooked you, which made you doubt reality, mathematical or physical laws, your sanity, memory or perception. The more improbable the better, but no second-hand legends please, share only what you personally experienced. If you had the event later explained rationally please use rot13 to avoid spoilers.