What is one fact that someone shared so confidently but turns out to be false?

My father was once driving with a friend when his friend, Dave, said, "Hey, look at that black Datsun."

My father immediately - and for no particular reason - lied, "All Datsuns are black. That's how they save money."

Years later, Dave wrote him a postcard with one sentence: I saw a red Datsun!

(yes, I'm aware of the Ford quote)