It is a historic artifact, not a design error. From the beginning of US coinage to 1891, silver "half dimes" were issued for the 5c denomination. (There were

It is a historic artifact, not a design error. From the beginning of US coinage to 1891, silver "half dimes" were issued for the 5c denomination. (There were also briefly silver 3c coins) The 5-gram base-metal nickel was introduced in the 1866 due to silver coins being hoarded amid post-wartime uncertainty, but eventually was preferred to the tiny silver coin.