On some guns it can cause breakage of the firing pin. It can damage the firing pin bushing. It can (eventually) let the firing pin bust thru to the chamber. It can peen the firing pin into the firing pin channel (making removal difficult to say the least). It can peen a big dent into the hammer. Every time you do it you run the risk of an accidental discharge. Or it can cause no damage of any kind and make you a better shooter.