What happened here?

My best guess would be acid or some kind of chemical. Doesn't look punched or drilled.

 

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I would guess a drill bit.

 

Hard to tell from these photos, but any acid strong enough to go through the copper would go through the zinc much faster. Instead of eating through both faces of the coin, it would go through one face and then go to work on the zinc core, splattering and foaming back against the first face.

 

Somebody got a drill bit set for Christmas, and wanted to try it out.

 

Combination hole and zinc rot. (rot has been cleaned out)