What is a Mule Coin?

By GovMint :

A mule coin is a fascinating numismatic error that occurs when a coin is struck using dies not intended to be paired. When this happens, a coin is struck with mismatched obverse and reverse designs, much like the hybrid animal from which it takes its name— the offspring of a horse and a donkey, a mule. Mule coins are highly sought after by collectors due to their rarity and the intriguing circumstances behind their creation.

Sources Used:

https://coinweek.com/mule-mint-errors-the-rarest-of-the-rare/