Washington Quarter Type B Reverse Error Question.

Technicly its called a mule which is when a die intended for one coin was paired with a die for another coin, it can be different finishes,denominations and some times countries
Some good examples of this are the 1999 and 2000 canadian mules with with missing dates and denominations,the 2010 canadian spec. dime with a circulation obverse and the 1985 Canada nickel dollar with a 1985 new Zealand obverse.
Hope this helps:-)

Ed. I have spent the last month or so buying the type-b reverse coins minted 1956-1964 at the philly mint. The story I read was the mint occasionally either accidentally or on purpose used the type b on circulation strikes (high relief) the standard type a reverse was typical for 1932-1964 p&d&s..CPG also points out some denver mint coins have been rumored to have the type b reverse. As far as errors on proof coins they exist. Doubled die Obverse and Reverse there was several die varieties known of they do exist. I recommend picking up the Cherry Pickers Guide books covers a very wide range of coinage. And gives cross referencing to other errors as well its a fun book. I'm sure other guys would recommend other books and websites. Speaking from experience you have come to the right place to learn about coins. There is some amazing information on this forum.