I could be wrong but in my opinion that is post mint damage, environmental corrosion or surface delamination. It's common on zinc copper coated pennies that have been in the ground, exposed to moisture, chemicals, etc. I don't think it's a mint error. The good news is it's worth one cent! The US Mint changed the metal content of pennies in 1982 from 95% copper to about 97% zinc with copper coating. These are prone to corrosion. Some collectors like to keep the older circulated copper pennies because their copper content is worth more than one cent currently. The melt value of a copper penny is about 2 cents. But it's illegal to melt them to make a profit.
Looks like a stain on the coin. Possible something was spilled/evaporated on it during its life. Not a plus for the coin. Just a distraction for the coin.
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