A "die error" coin means the coin is effectively a misprint, and though it made it into circulation it's a rare find and ...

One mistake made for the first time in 300 years caused 250,000 "mule" coins to become a collectors item, with the mistake sparking a eBay bidding war.

Some of the Royal Mint's Brexit 50p coins are considered rare as they have the incorrect date printed on them and were not melted down.