To me, it’s: That ancient people thought the Earth was flat.
We have records from around 430BC where Greek philosophers spoke of the Earth being a sphere. In 240BC the Greek astronomer Eratosthenes calculated the circumference of the Earth and was only about 2% out.
That’s a good one. I was thinking older history, but trickle-down economics has had a long enough run of endless disaster without any hint of trucking down for long enough now that we know it doesn’t work. If only we could keep it in the history books!