On rare occasions a double-struck coin will show that one of the dies (almost always the hammer die) rotated between strikes.  Such errors are most common on proofs from the late 1960's and early 1970's.  This 1966 nickel shows a rare business strike with such an error.  The hammer (obverse) die rotated about 15 degrees between the first and second strike.  This coin comes courtesy of Thomas Barosko.