Punk the tire

When I was commuting by road bike, rear tire blew out twice.  I always look for something sharp edge inside of tire when the tire blow out. But, I could't find it at first.
I started to run again after I fixed it. Later, it blew out. So, I disassembled rear tire again, and looked for edge inside the tire very carefully. And, I could find a sharp steel which cut by machine.
I should looked for it more carefully at the first time.