This looks like it could be a form of Lamination Peel. I am assuming this is a Lincoln Cent, but what year is it?
Well being a 1991 makes me rethink my answer. Being a Zinc coin with copper plating I would sooner think it is some kind of plating issue. It is hard to tell sometime with only a 1 dimensional photo to go by. Hopefully other will respond.
To me it looks more like a hole in the edge, and not a raised design feture as in a "Cud". Is it Raised or a hole?
Is the metal underneath dark colored? We may be back to my first thought. I have not seen or heard of the Zinc Cents having any Lamination Errors. But I do Believe it a posibility. It looks like there is a little depth to the hole, beyond what a plating issue might reveal.