Major mint errors on United States patterns are extremely rare. There are a few known broadstrikes, double strikes, off-centers and off-metals.
Pictured above is a unique and spectacular mint error, a proof pattern off-metal. It was struck on a copper nickel planchet instead of a silver planchet. It's composition is 76% Cu and 24% Ni, which is copper-nickel. It weighs .99 grams. PCGS certified it Proof-65. It is the only known copper-nickel Standard Silver Dime in the entire 1869-1870 series, from Judd #837 through Judd #872. The official Standard Silver Dimes were struck in silver, copper and aluminum.