There are many kinds of mint errors. Some make a coin valuable to collectors.
That said, most errors are not valuable, because they are too minor and/or too common.
Damage/alterations to a coin after it leaves the mint (called post-mint damage or PMD) doesn’t add any value.
Since there are a finite number of error types, but countless ways a coin can be damaged, the Rule of Thumb is, unless you can identify a specific error on your coin, assume it’s PMD.
Errors and Varieties although often used interchangeably, an Error is generally considered to be any coin as created by the mint that does not meet the specifications of the design and is not intended to be circulated. However, the term Error considered by many people to only refer to one-off issues which occur while a coin is being struck (see below: Strike Errors), whereas a Variety (see below: Die Errors) is an issue with the die itself, resulting in a production run of coins with the same attributes. Planchet and strike errors are typically worth less than varieties, unless they are substantial/extreme.
KINDS OF ERRORS & HOW THEY HAPPEN #
The best way to learn about errors is to learn about the minting process and how it can go wrong.
Briefly, a coin is made when a planchet (a blank piece of metal that is the correct size and shape) is struck (not “printed”) with dies (hardened pieces of metal that have the reverse of the design that is to be transferred to the planchet) in a coining press.
Errors can be roughly categorized as follows:
Planchet error: Any problem with the planchet, including incomplete planchets, wrong metal, wrong size, cracked, chipped, clipped, etc. These are one-off errors that can happen on just about any coin type.
Die error: Any problem with the die, like doubling of part of the design, a broken or chipped die, a die partially clogged with debris or grease, etc. Die errors usually occur in batches, with a series of coins struck by the flawed die showing the same error. These are often called varieties. When a variety has been discovered, verified, and catalogued, it’s called attribution.
Strike error: Any problem with the physical production of the coin in the press. Examples include off-center strikes, multiple strikes, rotated dies, misaligned dies, overstrikes, etc. These are one-off errors that can happen on just about any coin type.
Coins can have more than one type and/or category of error.
Here’s a more detailed description of the minting process.
Here’s a YouTube video showing how US quarters are minted.
And here is Wikipedia’s list of mint errors.
ERROR & VARIETY RESOURCES #
Here are some great resources to learn about, and see, different types of errors:
Error-Ref.com: A fairly comprehensive error/variety reference, containing explanations, examples, and attributions.
Variety Vista: Many attributed varieties on a number of US coin types.
Cuds on Coins: Specific to cuds and other broken die errors.
Wexler’s Die Varieties: Specific to doubled die errors.
The Lincoln Cent Resource: Covers all errors/varieties on US Lincoln cents.
Here are some references for specific, well-known errors/varieties:
US Cents:
Close AM vs wide AM reference 1
Close AM vs wide AM reference 2
Small vs large dates (1960/1970-S/1974/1982)
US Sacagawea Dollars:
2000-P “Cheerios” dollar vs regular
SOME NOTES #
Note that identification requests for error coins are no longer allowed. Please post to r/coinerrors instead.
However, here are some useful notes and links about certain error types:
A coin that is off-center (due to a misaligned die error, or an off-center strike error) is generally not worth any premium unless the strike is so offset that some of the design and/or text is missing. However, full dates tend to be more valuable than missing/incomplete dates.
Strike-through or filled-die errors, which result in light, or missing, text or design elements (e.g., the IN GOD WE RUST error) are rather common and are often of little value.
Most doubling on modern coins is machine doubling (“MD”) rather than the result of a doubled die (“DD”) error. MD happens when the planchet shifts while being struck, which is not uncommon. Unfortunately, the effects are somewhat similar.
Here’s a reference for telling a doubled die error from machine doubling.
Here’s a reference showing and describing other types of “worthless” doubling.
If you think you have a doubled die (DD) error, remember that nearly all instances of this type of error have been attributed — so check the resources above to see if you can match yours to an attributed DD error.