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.
This proof Bicentennial Dollar major mint error is unique in many ways and has no equals. It is the only known double struck with the second strike off-center. There are no off-centers known, no off-metals known, no mated pairs known, and no double denominations known.