For more info on the book, check out Coinzine!
http://www.coinzine.net/2010/09/sacagawea-mule/
The coin you reference wasn't an error. When someone intentionally
does something like that (strikes a copper coin in gold), it is
usually called a "piece de caprice".
The 1913 "V" Nickels are an example of a "piece de caprice".
BTW, the premise of the book you mention is a bit "iffy". The gamut
of error coins is very large, and what turns people "on" can be hard
to predict. And, probably, the very best error pieces are at least
semi-hidden because of fear of confiscation.
oly