It's not even a matter of being careful. I am extremely careful when I reload as noted by my fairly long (in my mind) track record without incident. Taking statements like "you can form your own brass" at face value is not absolute. And even checking each piece of brass before loading they pass. After loading not so much. And it only takes one problem to cause a major problem. I am fortunate that mine only cost me a bolt carrier group.

All things said I am still kind of pissed about it. Not that it helps. I just have to take measures to prevent it from ever happening again. No matter what.