There are 1965 quarters struck on the silver planchet as well as 1964s struck on the clad planchet,very rare though.
1776-1976 Bicentennial quarters with the S mint mark from the 3 coin proof set were struck in 40% silver.
Since 1994 proof quarters have been struck in silver with the S mint mark.
The U.S.Mint has minted billions of coins for other countries,it is possible to find U.S. coins struck on foreign planchets.
In 1963 while employed at Union Carbide Stellite,we rolled 2 different alloys of experimental metal for the mint,it is possible that coins struck on those alloys were circulated.
All manner of mistakes come from the mint,who knows?
hasbeen