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 collars and off-metals
have been known to be found in sealed proof sets. Proof errors are
aggressively sought after by many error collectors.
Images courtesy of Byers
(1969-S) Washington Quarter
Dollar (illustrated above) struck in conjunction with a Capped Die.
Discovered in a sealed 1969-S Proof Set.