Proof coins are struck by technicians who had feed the blanks into special presses. They are produced, examined and packaged using extreme quality control. It is very unusual to find major proof errors. A few broadstrikes, off-centers, double strikes in collars and off-metals have been known to be found in sealed proof sets. Proof errors are aggressively sought after by many error collectors. A very small group of Proof errors recently came from a collection that was auctioned by the State of California. The U.S. Secret Service inspected and released this collection to the State of California determining that it was legal to own. The State of California then auctioned the collection and it has been dispersed since the sale. This unique 1970-S Proof Quarter from the United States was over-struck on a 1941 Quarter from Canada. This mint error was originally discovered in group of San Francisco Proof Errors that was auctioned by the State of California. There is a significant amount of detail on both sides showing the design of the Canadian Quarter. This is one of the most fascinating and intriguing proof mint errors ever discovered.