I've been a numismatist all my life. never heard of fido in that application. The preferred term I've always used and heard is mint error.

As far as Fido being a traditional name for a dog...how'd that happen?

BTW, for folks so inclined, something to look for in today's pocket change...the US Maine state quarter is the lowest mintage of them all, and the one to save. Along with the New Jersey quarter because this is the only coin ever minted in US history with George Washington on both the obverse and reverse.