What's the cause?

Yeah, it's the pest asking more questions. Anyway, I tried looking this up but to no avail .

 

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That would be a post mint stain.

Any harsh or mildly harsh chemical can cause that. Soda, coffee, etc.

 

That's not being a pest.. that's asking a valid question. I do agree with paddy though appears to be post mint damage/staining rather then an error. I'd give it a drop or 2 of verdicare which would kill the verdigris (those green spots) and possibly lift that stain.. or maybe not.