Many types of mint errors are missed by inspectors and are inadvertently released into circulation. Clipped Planchets, Off-Center Strikes, and Wrong Planchets are just a few examples of the many types of error coins that slip out of the mint occasionally. Some errors are fairly common, but many are extremely rare.
1968-S Lincoln Cent. A proof that is struck on a dime planchet.
2001 Lincoln Cent struck over a Roosevelt Dime! The reverse under-type of the dime shows on the obverse and visa versa. An exciting Double Denomination error!
2002 Tennessee Quarter struck on a nickel planchet.
2002 Tennessee Quarter struck on a nickel planchet.