Some conspiracy theories turn out to be true. I don't have examples.
MK Ultra experiments turned out to be true.
The Dali Lama was paid millions by the US to disrupt Chinese culture.
Edward Snowden....self-explanatory
The real reason Anthony Bourdain killed himself was because Harvey Weinstein told him about his wife, Asia Argento grooming a 9 year old boy she work with on a film years ago, and maintained a relationship with until he was 16. He told him out of revenge for Argento pulling the trigger on the whole "MeToo Movement" against him.
The Gulf of Tonkin incident was staged so the US could start the Vietnam War.
During the 50s, cigarette companies convinced the public that smoking does not cause cancer.
Rachel Carson (who wrote "Silent Spring") was threatened directly by Monsanto, when she exposed the dangers of using DDT in public spaces. After Monsanto convinced the public that DDT was safe to use.
....when you speak about these things during the time it happened, back then you'd still be labelled as the crazy conspiracy theorists. The problem is, it takes years to prove it it's real.