A mule coin is essentially a coin that is a cross between two coins that already exist in history. One of the popular examples of this is the Sacagawea dollar reverse paired with the Washington Quarter obverse. Only 17 of these are known to man.
A coin press operator was mistakenly given an obverse die for a Washington quarter instead of the new Sacagawea dollar, and many thousands of these dramatic errors were struck before Mint employees noticed the mistake. Once discovered, employees culled out and destroyed all the muled coins they could, although several escaped into circulation.
This error coin was graded as an MS67 recently fetched $192,000, which was a new record for this type of coin.
Another popular error coin that has sold for thousands of dollars is the 1999 1 Cent Lincoln Cent Obverse Die muled with a Roosevelt Dime Reverse on a Lincoln Cent Planchet MS66 Red PCGS.