Looks like a counter stamp that was done after it left the Mint. That would not be an error nor makes it rare. Just damage. Not everything that looks strange or different on a coin is a mint error.
Here are some resources for counterstamped coins: Counterstamp Resources http://www.coinworld.com/news/us-coins/2016/08/lincoln-cents-counterstamped-post-mint-production.all.html# http://www.exonumia.com/art/cma.htm https://www.ngccoin.com/news/article/4878/Interesting-Counterstamped-World-Coins/ http://lincolncentsonline.com/Counter Stamped Cents.html http://numistories.com/537/counterstamped-coins-twice-the-history/
