Cracked piston in fresh engine

Oh here we go I just recently rebuild a 360 using KB 107 .040 over after gettin the engine running for the first time i found that the engine had a blowed head gasket on the passenger side. I replaced the head gasket and all was fine except a exhaust leak. Then one day the car started smoking badly and had way to much pressure under the valve cover so i pulled the engine thinking i may have had a bad ring only to find i had a small peace of my piston missing and a crack. What could of caused this it was only on #4 cylinder. I wanna know what the deal could be before putting this thing back together. Thanks to any one who reads or replys.