A batch of 20p coins accidentally issued with no date on them could be worth £50 each, say coin collectors.
Tens of thousands of the coins have been produced in error by the Royal Mint at Llantrisant, near Cardiff.
They are said to be the first undated British coins to enter circulation for more than 300 years.
The Royal Mint said the issue had been resolved but wanted to reassure the public that the faulty 20p coins were still legal tender.
But with experts saying the coins could be worth up to £50 each, anyone finding an undated 20p may be reluctant to put it towards a loaf of bread.
The date on the new 20p was moved from the 'tails' to the 'heads' side when the country's coins were redesigned last year.
The coins without a date were created by accidentally pairing up the new 'tails' side with the old 'heads' - meaning no date appeared at all.
A batch of somewhere between 50,000 and 200,000 of the coins are thought to have entered circulation.
A Royal Mint spokesperson said: "The Royal Mint can confirm that a small number of new design 20 pence coins have been incorrectly struck using the obverse from the previous design, resulting in these coins having no date.
"The issue has now been resolved and the Royal Mint would like to reassure members of the public that these coins are legal tender."
This is what one will look like:
The odds on finding one are small, but still worth keeping an eye out. The London Mint Office is offering about 50 quid, however on ebay you could get A LOT more - apparently one went for about 400 quid yesterday!
unfortunately I am not that lucky - but it does not hurt to keep an eye out. ESPECIALLY if you work in a place that handles a lot of coins.
"For too long, we have been a passively tolerant society, saying to our citizens 'as long as you obey the law, we will leave you alone'" - David Cameron, UK Prime Minister. 13 May 2015.