Many coin enthusiasts collect mint errors. These are coins that bear some sort of flaw acquired during the minting process. In general, errors are classified into three categories: planchet preparation errors, occurring when there is something wrong with the blank metal from which the coin is struck; strike errors, which occur when something goes wrong with the striking process; and hub or die errors, occurring when there is something wrong with the die itself.

Hub and die mint errors are generally more collectible than the other types of errors. The hub and die errors generally result in many coins all bearing a similar flaw or error, whereas planchet and strike errors are more typically one-off occurrences. In modern coins especially, a single hub or die error might result in thousands and thousands of error coins.

In the modern coining process, a die is produced from a “hub” which bears a raised image of the design. Dies are heated (to make them softer) and pressed against the hub to receive the image. The die bears an incuse image, which is then struck against a metal planchet to produce the coins themselves, which thus have a raised image (like the hub).

As minting technology has improved, modern dies are usually created with a single impression from the hub. However, before the 1990s dies almost always required multiple impressions to transfer the design sufficiently. A doubled die error is a specific type of die error occurring when the hub used to create the impression on the die is not perfectly aligned for each impression. As a result, the coins produced bear a design that appears to have been struck twice (“doubled”). In reality, however, the coins are struck properly, but with a die bearing two slightly overlapping impressions.

One of the most famous mint errors is a doubled die error: the 1955 “doubled die” Lincoln cent. This was created when an obverse die was misaligned on the second impression from the working hub. In this particular case, the doubling is most noticeable on the date and inscriptions rather than the image of Lincoln.

All of the 1955 doubled die cents were produced in a single night at the Philadelphia Mint. At least 20,000 of the error coins were released into circulation. Because almost all were discovered after they had been circulated, most show signs of wear, and high-grade mint state coins are very valuable. The 1955 doubled die cent has also gained notoriety from being featured in various movies and books.