Not too common, but it doesn't add any value to the coin unless there's some cool story behind why it's on there. My guess? Some kid bored with a punch that had that symbol.
The others are correct.. the correct term for this is a Counterstamp , Countermark or Punchmark.. not an error but PMD - Post (after it left the mint) Mint Damage I want to share this webpage with you - https://en.wikipedia.org/wiki/Countermark Here is another example -
