Carrots help you see in the dark
Everyone knows carrots help you see in the dark.
Except they don't.
In World War 2, the Brits invented radar, and used it to their advantage during night flights. The Germans were wondering how the Brits could "see" so well in the dark, so to protect their new secret advantage, they started leaking stories in the media about how carrots help you see in the dark, and how the RAF pilots were consuming lots of the vegetable.
More details at Snopes, the debunker of myths.