I agree with Chris. Lamination, and a big one at that. It amazes me that some of these Wheatie Errors get so much circulation wear before being discovered. Can you post a photo of the obverse?
That's what I thought at first also but I didn't think wheat penny's could delaminate because there solid and not laminated like a zinc.
SuperDave gave me a really good explanation when I asked the same thing. Thanks SuperDave! https://www.cointalk.com/threads/1942-lamination-error-penney.297142/#post-2752513
Your right, that is an excellent explanation. Thank you and SuperDave! I would think than that there fairly rare in copper coins.