Yea, it's really hard to tell. Could be Environmental or a Plating issue. Maybe? In the end, it's Not an error of any kind. Sorry.
Plating issue, very common, no value beyond face. The Mint hired contractors to make the planchets for Lincoln copper-plated zinc cents and they didn’t make them very well at the beginning. Some were very grainy looking (like this one), some had large pellets or blobs scattered on one or both sides and some had the plating too thin or even missing completely in spots. This went on, off and on, for over 10 years and still happens today with Shield cents. As you can see, The Mint used them anyway.