Error Pattern coins are extremely rare. There are fewer than 2 dozen confirmed errors over the course of the entire pattern series. One of the earliest is shown here. It is an example of J79a, showing the popular seated liberty obverse here dated 1838 combined with a flying eagle reverse similar to that used on the 1838 pattern dollars.
This piece shows about a 20 degree rotation between strikes on the reverse. This is most noticeable between the eagles’s tail feather and the letters “AR” of “DOLLAR” where the original striking of the tail feather and talon can be easily seen.
This, one of about a dozen known, is the Mint Collection specimen presently in the Smithsonian Institution.