Our regular members will know that when I comment on error coins, I very often cite Occam's Razor as my rationale for condemning something as "not an error"... but I'm thinking this is an excellent application of Occam's Razor to prove a coin actually is an error instead.

Scenario 1: the coin really is a nickel struck on a penny planchet.
Scenario 2: the coin is a normal nickel, which someone has neatly trimmed or lathed to match the weight of a penny, and then the coin has acquired environmental staining to make it appear coppery.
Scenario 1 is clearly the simplest explanation in this instance, so is therefore the most probable.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis