What’s the biggest piece of propaganda you believed but later found out was not true?

I don't know if I'd call it propaganda, but I found it interesting how I could have believed that Manson's ambition for a race war was proven for such a long time. The reason why a popular deception like that would be floated for so long is what's most interesting.
Then there's the exaggeration of the Spanish Inquisition and just how many fell for that hook, line and sinker is also an interesting case study