We've found that you can over-grind a valve and seat, even if you only use a polishing or lapping compound. Then the edge of the valve becomes too thin and begins to warp away from the valve seat. The rim of the valve can become so thin that the valve starts to become more and more recessed into the valve seat area, finally becoming small enough that it's well inside the port area. The engine is ruined at that point, and a new cylinder head and new valves are needed.