probly brick backstop.........1st basemen was picking his nose......ball kicked hard off of backstop and rolled past first base.......ditto to third base
or
the catcher in his haste to throw the ball back to home unnecessarily throws it hard and as harper was rounding 3rd......the ball goes into centerfield so he scores........
lost of weird ways for him to score from 2nd.................no way in hell he scores legit without an error though