It's emerged that many of the UK's fancy new £1 coins are defective. While they might look odd, coins with minting mistakes can be worth a pretty penny. Read on to discover what other mis-struck coins you might find in your change that could make you a mint.

In 2000 the Royal Australian Mint had a bad run with some 20c coins. They were stuck incorrectly and many feature tilted effigies or small errors, particularly around the rim. The example pictured is more damaged than most – if you look closely at the rim on the face side, you’ll see how off center it is. Similar coins have sold on eBay for over AUS$10 (US$8/£6) – not bad for a 20c piece!

This 2000 US penny has been struck off center, giving it a thicker top left-hand corner rim on the ‘tails side’ and almost erasing part of the ‘In God We Trust’ inscription on the ‘heads side’. It is a fairly common error, which means you may have one lurking in your change right now. If you do, you can sell it on eBay for around $13 (£10).

Canada stopped making copper one cent coins in 2012 and took them out of circulation. Given that this penny is dated from that year, and is clipped on the edge on both sides, it could be the last of its sort ever made. Coinscan.com estimates its value at CA$25 (US$18/£14), not bad for a run-of-the-mill penny!

In 2005 the UK Royal Mint released a commemorative £2 to mark the 400th anniversary of that fateful 5th of November when Guy Fawkes tried to blow up the Houses of Parliament but was caught and jailed. The Mint was trying to be clever by printing the famous verse on the edge, but the ribbing on the coin clashed with the Rs. The coins appear to read “Pemember, Pemember the Fifth of November” instead of “Remember, Remember” – now they fetch up to $19 (£15) on eBay.

Something went terribly wrong in the minting of this Peruvian sole. It appears as though the coin slid off center before it was struck, giving it this misaligned appearance. What is remarkable about this example from 1976 is how far off center this coin is. It might even be unique, and is worth around $20 (£16) today.

This particular coin is extremely rare because it features a combination of two errors. It features a partial brockage, which is when a coin is struck against another coin, and it has been struck twice, causing some features to be doubled. According to Australian-coins.com either of these errors are unusual in Aussie coins, and both together is a true rarity.

In 2015, the UK Royal Mint launched a WWI Royal Navy design for the £2 coin. Shortly after, it was noticed that some of the coins appeared to feature a waving flag on the top mast of the ship design, while others didn't. The Royal Mint insists that it’s not a design error, but that occasionally debris can get caught between the die and the coin while striking, according to the Change Checker blog. It’s a simple mistake, and these coins fetch around $26 (£20) on eBay.

At the end of 2008, the UK Royal Mint admitted to producing around 250,000 20p pieces with no date on them. The error occurred because an old ‘heads side’ was used with a new design ‘tail side’ which meant there was no year on either side. There are still quite a few of these in circulation, and if you find one they could be worth $260 (£200).

In coin jargon a mule is a coin struck with the wrong die (stamp). In 2014, around 17,000 commemorative £2 coins for the ‘Year of the Horse’ were struck with the wrong die by the Royal Mint, meaning they were lacking its cogged or denticled rim design, and featured a smooth rim instead. Some 38,000 of these coins were struck in total, so it was a big error. These coins are highly sought after, and can be worth as much as $226 (£175).

This Canadian (loon) dollar was found in some change from a restaurant in Quebec, according to coinscan.com. The tails slide “appears to be struck through a late stage cap”, they explain. This sort of error is very rare in a modern coin, and not many of these sorts of coins have been discovered. They estimate one such coin to be worth CA$500 (US$366/£388).

Before 1981 all 2p coins had the words New Pence on them, rather than the current Two Pence. But in 1983 the Royal Mint accidentally struck a small batch of the new coins with the old New Pence design on them. They can be worth up to $515 (£400)! But beware: some eBay sellers are trying to pass off standard 1981 New Pence coins as the rare 1983 mis-struck ones, and those are just worth their face value.

This 2000 $1 coin might look normal but if you look closely at the circular rim of the coin on the Queen side, it appears doubled. That's because this dollar was minted with a 10 cent die. “The mule was made when a technician at the Mint in Canberra accidentally paired the mob of 'roos dollar reverse with the Queen's head obverse, normally used for the 10 cent piece,” according to the Australian Coin Collecting Blog. If you've spotted one of these, it could be worth AUS$1,000 (US$753/£584).

This coin currently available on eBay might well be unique. A mint condition 1999 Connecticut Quarter featuring a charter oak can be worth as much as $50 (£39) at auction, according to Coin Trackers. This one has been struck three times, meaning it has a very unique engraving on both sides. It is on the market for $899 (£696) and could go up in value over the years.

A modern coin is made by striking a die (a type of mold) onto the metal a few times. In Philadelphia in 1955, the coin base was not secured between blows, causing a double print in an estimated 20,000. The double die is most visible on the bust of Lincoln and on the date. If you find one in your gran’s old coin jar, keep it! They can be worth over $1,000 (£776) on auction sites today.

To celebrate the London 2012 Olympics, the Royal Mint released a series of 50p coins with 29 different designs. A limited edition commemorative set of 29 goes for around $52 (£40) on eBay. But the original swimming coin is worth over $1,290 (£1,000) on its own. That’s because of a minting error that caused the swimmer’s face to appear covered with water. Only a few were made and pulled from circulation quickly, but some are still out there, so check your pockets!

Several 1971 'silver' 2p pieces have been discovered lurking in small change and have fetched up to $1,741 (£1,350) at auction. UK 2p pieces are usually copper colored, but in this instance copper-nickel rather than copper blank was left inside the minting machine by mistake, producing a small number of silver coins. Luckily quality control never noticed the error, but a rare 'silver' 2p might still be lurking in your change today.

These coins caused a stir in 2002 when, not long after the introduction of the euro, Italian authorities became aware that one batch of coins was the size and shape of the 2 cent coins, but had the printed face value pattern of the 1 cent coins. Italian coin collectors Bolaffi acquired a bag of such hybrid coppers, but were sued by the Italian Mint, who claimed Bolaffi had no right to the faulty coins. In the end the collectors prevailed and one of the coins sold for €6,600 ($7,206/£5,590)!

According to Mint Error News, four Sacagawea commemorative dollars have been found with transitional errors. These occur when a coin is struck with a different metal composition from the past then fed into a coin press for the current year. According to the site, these rare commemorative coins can be worth up to $15,000 (£11,600), and there may be more of them out there!