I have been using firearms since I can remember (and I'm about to turn 60.) My dad taught me safety with firearms and hunting beginning when I was a child, he was career Army. I've carried a firearm for my jobs since I was 21. During this time, and since I've retired from LEO and security jobs I've done handgun competitions, historical reenacting and live fire demos, and have a range in my back yard. I have never had a an accidental discharge nor any other problem. But, and but is a bad word, somehow this occurred the other day and I still can't figure how I did it. I occasionally take my CCW to the back yard to shoot the carry ammo out of it. This is a S&W Shield which I carry 95% of the time. I practice a lot with it, but use FMJ practice ammo when I do. I carry Hornady Critical defense and about every 4 months or so I shoot it out and put in new rounds. This day I did my usual, and when I pulled it and shot at my target, it jammed on the third round, it did not feed. I tried to eject it with no success and do the rack, smack drill, but had a time trying to clear the gun. I was worried that something was wrong with the gun or the magazine, so I picked up the unfired round and put it back in the magazine. The gun wouldn't chamber. When I looked closer, it was a Hornady .40S&W. I do own a .40 Sig and use Hornady in it, but I keep my ammo totally separate in their packaging and away from the table when I'm cleaning a firearm. I have NO idea how that .40 got mixed in with the 9mm. I was also surprised how it fed into the magazine as easy as the 9mm. It shook me up, thinking about carrying it in that condition for the last three months and if I needed it, I would have had a major problem. My point is, I guess you can think your the most careful, safety minded individual out there, and something can still go wrong. I will say, I don't take it for granted now, I look at the case head every time I load a cartridge in a magazine. P.S. I do know that I bought some federal .38 special a few years ago and there was a .357 in the box, but I'm sure this was my fault. Beats me how I did it.