I left this post over in LCR and since the same topic appears here, I thought that I would let you all know what I think.
BJ Neff
Mule coins are indeed rare and at times not made by accident. One of the more widely known mules is the quarter obverse on the
Sacagawea dollar reverse. To date, ten have been found of this mule with three different die pairings. This unusual amount of die pairings suggest that while the first may have been an accident, those that followed were not.
So, how does this happen. We can go back to the 1857
Flying Eagle cent for a partial answer. During that year, three mule die clashes occurred involving the
Flying Eagle cent, the
Seated Liberty quarter and half and the 1857 Double Eagle gold piece. While the most popular story surrounding these clashes was that the nightwatchman did it, that was not the real story. Chris Pillod analysis of the clashes showed that they occurred during the changing of the dies.
What this points to is that these mule coins were more than likely produced during the changing of the dies and probably not the result of just forgetting that the dies were not matched.
BJ Neff