The Royal Mint has issued up to 200,000 20p coins without a year date - making them worth £50 each.
In what has been described as an "incredibly rare" error, the coins are missing their 2009 stamp and are instantly valuable - and could become even more so.
The last time an undated coin entered circulation was more than 300 years ago in 1672 when Charles II reigned.
The mistake happened last year when all our coins from 1p to £1 were redesigned.
The designs for the 5p and 10p had remained the same for 40 years with an unwritten convention images should be updated every four decades to keep them "fresh".
The updated 20p had the date shifted to the head's side. But between 50,000 and 200,000 coins were minted using the old version of the Queen's head which does not have the year date.
Now, The London Mint Office - a private company with no connection to The Royal Mint - is offering lucky owners of the odd coins a chance to cash in.
People with an undated new 20p need to register their details with The London Mint website - www.undated20p.com - before sending in the coin and claiming their £50 reward.
This is not the first time Royal Mint has made a mistake. In 1983 a new 2p was struck with the wording "new pence" rather than "two pence".
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