closeup of 2006 canadian pennyError coins—what are they, exactly? Simply put, they are coins that were manufactured incorrectly at some point in the minting process. These mint errors can be wide-ranging, with some examples being off-center striking, double striking, indents, or even improper metals used. Depending on the error, error coins can still find their way into circulation, and then eventually into the hands of coin collectors who treasure them for their unique qualities. As part one of a two-part series, here are four major error coins from around the world that have become valuable to coin collectors.

2014 Britannia 1-Ounce Silver £2 “Mule” Coin (UK)

The 1-ounce silver £2 Britannia coin of the United Kingdom is a bullion coin—that is, a coin struck from a precious metal and saved as an investment rather than used in everyday commerce. Minting of the coin in 2014 produced about 60,000 “mule” coins, which are coins that feature mismatched obverses and reverses. This mule coin comes in two varieties because of two particular mistakes that were made: first, 1-ounce silver £2 coins struck with the obverse intended for the Royal Mint’s Year of the Horse bullion coin; and second, 1-ounce silver £2 coins struck with the reverse intended for the Royal Mint’s Year of the Horse bullion coin. This coin remains a relatively unknown error coin, and several collectors purchased them at face value. Only time will tell how much these coins will eventually sell for.

1966 “Wavy 2” 20 Cent Coin (Australia)

This particular error coin features an imperfection in the baseline of the large 2 on the reverse and is considered to be worth $100, or even as much as $5,000 depending on the quality. The error coins were created when just one of the dies used to create the 30 million coins was different, featuring a wave in the baseline of the 2 that mimics the waves of the water above the baseline.

2006 Magnetic “No Logo No P” Penny (Canada)

One-cent coins minted in 2006 were minted on non-magnetic planchets and on the obverses featured either a P or a small logo below the effigy of the Queen of England. Some coins, however, were minted on magnetic planchets and bear no P and no logo below the portrait of the Queen. These coins are estimated to be worth as much as about $650 each, depending on their condition. One main found about 500 mint version of these coins in his penny collection and could potentially make over $300,000 in selling the coins.

1983 "New Pence" Twopence (UK)

In 1971, new twopence coins were introduced as legal tender in the UK, with the new coins featuring the inscription “new pence” on the reverse to distinguish them from older coinage. This inscription was removed in 1982 and replaced with the inscription “two pence” on these coins. Therefore, all pre-1982 twopence coins should feature the “new pence” inscription while all post-1982 twopence coins should feature the “two pence” inscription. In 1983, however, a mistake led to a batch of twopence coins being printed with the old “new pence” inscription. These coins are valued today between $750 and $1100 each.