All the recent talk about conspiracy theories reminded me of the book
Foucault's Pendulum.
It was a complicated story but IIRC, it was about three publishers who were amused that their clients were so gullible. So they decided to invent their own fake conspiracy theory just for fun. They even used a computer to make up random parts of the theory. They thought it was all harmless and funny because they knew it was fake.
It all went south when their gullible clients started taking it seriously, turning their fiction into a thing of deadly real-life consequences.
Good read.
Edit: Fair warning: there are many references to occult subjects.