There are billions of coins worth far more than the price indicated.
Some may be because they were produced in small numbers or because they had errors or imperfections.
And if you find one that is wanted by collectors, you could make a small fortune.
A $ 50 coin was sold on eBay for £ 3 & # 300; and it's likely you've had a similar bag in your pocket at some point.
Known as the Dictionary Coin, this coin was released in 2005 on the occasion of the 250th anniversary of Samuel Johnson's book, which was used by such famous authors as Charles Dickens, Thomas Hardy, and Oscar Wilde.
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It displays the word fifty in the style of the dictionary entry.
According to Mirror Online, Northampton-based eBay user ckyp2009 listed the coin under the Buy It Now option on the website, and there are currently seven people watching it.
The description reads: "Circulating commemorative coin 50p. Only a limited number was produced."
However, the coin is not so rare as it is listed as "general" in the Change Checker manual.
The Royal Mint made 17,649,000 of these coins – compared with 22.7 million of the Royal Arms in 2008.
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The rarest 50th in circulation is currently the Kew Gardens coin, which was issued in 2011 for the 250th anniversary of the Royal Botanical Gardens.
Only 210,000 were ever produced and are now selling online for large quantities.