A 'silver' two pence piece dropped in a charity box last year has sold for £1,350.
The rare coin was found in a Royal British Legion Poppy Appeal tin in Malmesbury, Wiltshire, and was initially believed to be a fake.
But when volunteers took it to a bank to be destroyed, staff suggested that they send it to the Royal Mint - which identified it as a rare coin made in error.
A nickel-plated steel blank, which would normally be used to make a 10p piece, somehow ended up being struck between 2p dies.
The coin has now been sold to The Westminster Collection, a company which specialises in buying and selling collectable coins and stamps.
The coin isn't completely unique. Two years ago, a similar silver 2p piece was sold for almost £1,400 by a former petrol station owner who'd found it in a roll of coins from the bank back in 1988.
And last summer, another charity benefited from the proceeds of a similar coin. That, too appeared in a collecting tin - in this case for the Royal Berkshire Hospital charity.
And when it was spotted by volunteer Becky Jennings, she put it up for sale on eBay - and netted more than £800 for the charity.
The Royal Mint recommends keeping your eyes peeled for 50p coins issued in 2009 to mark the 250th anniversary of the Royal Botanic Gardens in Kew. Only 210,000 were made, and they've sold for as much as £310.
The second rarest coin design in circulation is the 2002 Commonwealth Games £2 with a Northern Ireland flag, of which only 485,000 were made.
Third, fourth and fifth are the 2002 £2 coins featuring the Welsh, English and Scottish flags respectively. The rarest £1 coin is the 2011 Edinburgh £1, of which 935,000 were made.