A coin used during the Nagasaki trading era (1659-1685)

What kind of error do you call this? Double struck? Misstrike?

Of course I have no good knowledge but based on the pic, the coin may have been sitting in a pile with other coins of other sizes. Maybe the oxidation of one smaller coin contaminated yours because they were on top of each other in a pile for years.

Know what I mean? I'm really tired.

I may be mistaken if there is any sort of relief or dimensionality to the coin where the "double struck" comes in.

Seconded.

Being a cast coin it would rule out a "strike" error.

-Is the 1600s really ancient? The Romans must be pre-historic, then!

These coins were not struck, but cast in molds. Iron examples were known as tane or seed coins and are found occasionally. The seed coins were used to make the molds that the bronze was cast in. My guess is that the tane was misapplied in this case, leaving a mark in the mold for the next coin to be cast. Japan used this method of coin manufacture for bronze Mon coins until the 1860's when everything was completely modernised and monetary reform was instituted.

I don't believe the "error" is due to corrosion damage.

Here is another better quality image:



And with the same image under negative filter,



It rules out intentional damage I think.

Scottishmoney is right - I wasn't thinking about the minting technology back then. Would the proper technical term for this be mold casting error?