Feeder fingers feed the un-struck planchet into the coining chamber but sometimes they malfunction or get jammed and get struck by the dies, or are struck through a coin. Above is an example of a Sacagawea struck through a feeder finger and then an actual feeder finger struck with the hammer and anvil dies.
The dime example above sold for over $5,000 and the Sacagawea above sold for around $300 at Heritage Auctions, so these are rare and valuable mint errors.