With the phasing out of the round pound coin, there's been a new interest in collecting coins, with many people aiming to collect all 12 versions.
Some rarer types can be worth quite a bit, as can limited editions of other coins.
But one of the simplest ways to be sure you've got a coin worth more than its face value is to find one with some sort of error.
Most mistakes by the Royal Mint are spotted before the coins are released, so so-called error coins are pretty rare. We look at a few that are known to be in circulation.
The least common (and most valuable) £1 coins
The undated 20p
Back in 2008, every coin from the 1p to the £1 was redesigned using a new die with the date on the reverse. However, when it came to the new Royal Shield 20p, the wrong die was accidentally used, meaning the coins carry no date at all. In perfect condition, they're worth up to £200.
10 surprisingly valuable coins in your wallet: the ones you don't know about
The 'silver' 2p
Last year, a 'silver' 2p made headlines after it was dropped into a Royal British Legion collection tin. Somehow, a 10p 'blank' had been used. Only one other similar coin has ever been found; both sold for over £1,300.
In 1982, the Royal Mint decided to change the words 'new pence' to 'two pence' - but somebody didn't get the memo. In 1983, a small number were released with the old wording; and they can be worth up to £400.
The 'inverted effigy' £2
Every now and again, a die will work itself loose and coins will be stamped out of alignment. Around 1 in 200 Britannia £2 coins have the Queen's head almost upside down. In good condition, one of these coins can sell for over £100.
Do you have one of these valuable £2 coins?
The Battle of Britain 50p
The Brilliant Uncirculated 50p was issued in 2015 in presentation packs - but was struck without the denomination in either numbers or writing anywhere on the coin. Technically, this isn't an error, as the Royal Mint says it was deliberate, but it does make the coin pretty rare. However, while they did change hands for large sums at one point, they're now worth just a couple of quid.
Upside down edge lettering
This is an error that can occur with any coin that has writing around the edge. The reason is that this lettering is printed before either of the faces - and sometimes a coin goes in upside down. Their value varies.