Mint-made errors are errors during the minting process. Groups of coins with distinctive characteristics are known as varieties. The term variety applies to coins with both intended and unintended differences while the term error refers only to coins with unintended differences. Nevertheless, not all errors are varieties. Although there may be many identical examples of some errors, others are unique. For example, there may be many indistinguishable examples of coins with a specific die crack, while off-center strikes tend to be unique. Being unique does not mean that an error is valuable.