Couple of situations for you.
4th and 2 on B's 45. A12 runs a quarterback sneak; the wingman comes in selling his spot, which ends up as near to nose on B's 44 as makes no difference. The officials determine that it is short and the ball goes over on downs. Where do we have 1st and 10 from? Do we leave the ball where it was, so it will start tail on new-A's 44? Or does the umpire pick it up, clean it off, and then replace it so that it will be nose on new-A's 44?
As before, except this time it is 4th and goal, and the spot is either nose on B's 1, or middle between B's 1 and the goal line. Leave the ball, or clean it off and put it on new-A's 1?