In 1844, Sir David Brewster discovered an iron nail in a block of stone in Scotland’s Kingoodie Quarry. The nail was embedded in a Cretaceous block from the Mesozoic era; in 1985, the British Geological Survey dated the bed at between 360 and 408 million years old.
An iron nail has no business in the Mesozoic era, and no ordinary nail could avoid oxidation for more than 400 million years.
So how’d it get there? No one knows.