Proof coins are struck by technicians who hand 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 collar) and off-metals have been known to be found in sealed proof sets. Proof errors are aggressively sought after by many error collectors.
Pictured above is one of the most spectacular Proof Ike Dollar Mint Errors of all time. Not only is it a spectacular double struck, it is also struck on a clad half dollar planchet.