I think "birds migrate" is a good contender for one of the most startling "humanity discovered that when? and they thought what instead? And it took what to disprove it?" factoids: https://en.wikipedia.org/wiki/Bird_migration#Historical_view... https://en.wikipedia.org/wiki/Pfeilstorch

It seems so simple and common sense and everyday. It doesn't involve microscopic lifeforms or planets or stars, just... birds. "Where do birds go during the winter? Well, birds are good at flying away from hunters or other bad things. That's why they fly. So maybe they just fly somewhere else. I look up in Fall and I see flights of birds flying in a specific direction. And people in other areas say birds arrive from elsewhere for a while. What else would they do? Turn into little fish and hide in the water all winter, or spontaneously regenerate out of random little clams? Anyway, here's a bird with a big old Central African spear stuck through its whole body who managed to fly here alive anyway which proves that."

There's a creepy Japanese urban myth about a guy who injured his knee at the beach. He developed knee pain, so he went to the doctor, and they discovered barnacles growing inside his kneecap. A goose appearance would probably make it goosebump-less.

There's a good clicker game based on this https://everest-pipkin.com/barnacle-goose/

Myth or no myth, the barnacles are delicious

(when fresh)

I fail to believe outside of when we lived liked apes without any reading and writing this 'myth' was believed.

People had chickens and ate bird eggs, people would eat goose barnacles and not see baby geese, it'd be like all the other barnacles.