THIS IS A RARE ERROR OF THE 20 PENCE... FIRST ONE THAT WE HAVE SEEN.. DETAIL FROM THE SUN NEWSPAPER, WHICH YOU CAN GOOGLE INFORMATION. THIS IS WHAT THEY SAID.
Don't mistake a rare bronze 20p coin for an old and dirty one - it could be worth a fortune. One of the rarest coins ever, the bronze 20p coin.
After collector David Crosier found the coin, the Royal Mint confirmed it was a mistake. It said it was the result of an extremely rare minting error, with a blank 1p somehow finding its way into the presses and ending up with a 20p struck onto it.