UK shoppers are on the alert to check their pockets and purses for a £5 note that could be worth over £100.
A fiver could be worth over twenty times its value if proven to have special details.
Features to look for include an unusual serial number, errors or just simply by virtue of being rare.
Special notes like these are going for a tidy sum on websites like eBay, the Sun reports.
Recently, a 64-year-old five pound note in good condition from 1957 attracted 28 bidders and was sold for £113.11.
The note from 1957 was unique for its jumbo dimensions that were unique to the time period.
The largest £5 notes ever were in circulation from 1945 to 1957 at 211mm by 133mm.
For reference, they were 1.69 times longer and twice as tall than the contemporary fiver which is 125mm by 65mm.
A modern, polymer £5 can also be worth far more than its face value provided it has something that makes it valuable to a collector.
For example, one recently sold for £99 because it had a serial number of AA01 010110.
This indicates it was one of the first polymer £5 notes to be made.
The first 'plastic fiver' with the serial number AA01 000001 was ceremonially presented to the Queen.
There are just 999,998 other fivers left with the prefix AA01.
This sounds like a lot, but it is actually rare, as there are more than 400 million £5 notes in circulation.
Another serial number that makes £5 notes more valuable is instantly recognisable - AK47.
Banknotes starting with this prefix are worth more to collectors, as the number is the same as the model of the famous Russian assault rifle.
Some have been listed on eBay for up to £160,000, but most sell for around £100.
Fivers can also be worth more if they contain printing errors, says the Mirror.
One sold for £266.05 online this year because it had no signature from the chief cashier of the Bank of England - normally a security feature on all banknotes.
But errors in production also make coins more valuable too.
The reason why they’re often sought-after is because they are usually hard to come by - in some cases, they are even a complete one-off.
However, there are some well-known errors where larger batches of coins were released into circulation after being minted with a mistake.
After all, the Royal Mint manufactures between three million and four million coins a day - so it’s easy to see how mistakes can happen.
But fakes are also out there - so be aware of scammers trying to flog counterfeit pieces online.
One great example is a 2p that sold for an astonishing £1,357.
The reason is it was minted using cupronickel - the silver metal alloy normally used to make 5p, 10p, 20p and 50p coins, and not the usual copper alloy.
If you happen to find a so-called "silver 2p" the rough guide price you could expect to sell it for is £600, according to Colin Bellamy, the man behind Coin Hunter.
However, one of these error coins previously sold for £1,357 after former petrol station owner David Didcock discovered it among a roll of brand new pennies while filling up a till in Poole, Dorset, in 1988.
