While the U.S. Mint is very careful to produce all coins within strict quality guidelines, the Mint isn't perfect. Although many different mint errors are known, they can be cleanly broken down into three categories - planchet errors, die errors, and striking errors. Some examples of each are:

• Planchet Errors: Any error caused as a direct result of how the planchet was made. These include clipped planchets, tapered planchets, wrong stock errors, wrong planchet errors, sintered planchets, and others.
• Die Errors: Any number of errors caused by the die that mints coins. Examples are broken die (or CUD if at the edge of the coin), clashed die, damaged dies, and others.
• Striking Errors: A large number of different errors caused directly by the action of striking coins. These include, but are not limited to, off-center struck coins, broadstruck coins, partial collar errors, indents, chain strikes, saddle strikes, ram strikes, and others.

Some errors are very rare and valuable, and others are rather common and have little value. A general set of rules to follow when considering errors and their value:

• Older error coins, especially those from the 19th century and early 20th century are generally far scarcer than more modern error coins of the same type and will command substantially higher premium values.
• Errors that are very obvious, especially those that are larger than a normal coin of the type, are generally worth more than errors that take concentration to notice or will fit in a normal roll of coins of the type.
• The more unusual an error is generally the more premium it will command.