This error occurs when a planchet receives impressions from dies of two different types (designs) of the same denomination.  The most spectacular example of this error is a 1970-S Proof Washington Quarter struck over a 1900 Barber Quarter (70 years apart, struck at two different mints, in the wrong metal, first for circulation, then as a Proof)!  The only other example of this error is a recently discovered 2000-P Massachusetts 50 States Quarter struck over a 1999-P Georgia 50 States Quarter.

While the status of the 1970-S/1900 Quarter remains questionable, we're not surprised to see the Massachusetts/Georgia error, especially in light of the spectacular mistakes made by the Mint in recent years.  However, the new discovery becomes doubly exciting because production of the Georgia and Massachusetts 50 States Quarters was separated by over 70 days - the period during which the Connecticut 50 States Quarter was being produced!  To accept this new dual type error, we must believe that a Georgia Quarter remained stuck in a hopper for nearly 2-1/2 months before becoming mixed with blank planchets and then struck as a Massachusetts Quarter.  Fortunately, we have precedents in such coins as the 1943 Copper Cents and the many Wrong Planchet/Wrong Metal error coins to assist us with this leap of faith.

According to error specialist Mike Byers, a woman spotted the new Massachusetts/Georgia error in circulation while sorting through her States Quarters.  The coin changed hands several times and finally found it's way to ANACS where the coins was authenticated, certified, and graded MS-64.

The U.S. Mint has produced nine different States Quarter designs over a one and a half year period, with almost nine billion produced. This is the first and only known example of a dual date and dual state error.

Images courtesy of Byers Numismatic Corp.

Significant examples:
2000-P Massachusetts 50 States Quarter Dollar struck over a 1999-P Georgia 50 States Quarter Dollar, ANACS MS-64.  Found in circulation - various intermediaries - Byers Numismatic Corp.  This coin was offered by Byers Numismatic Corp. on eBay between September 8-18, 2000 (Item #432372001) at an opening bid of $12,500.00 (no reserve).

Sources and/or recommended reading: