I see people here acting like they can't make mistakes. You can. I have seen it happen to people a lot more responsible than me. All it takes is a little distraction - internal or external.
There are no unloaded weapons.
Back during Operation Desert Storm, the only casualty suffered by the Kentucky National Guard was a major who was twirling his 9mm on his finger. The pistol went off and hit him in the thigh just above the knee. The damage was so extensive that the leg had to be amputated.
About three years ago, one of my friends was out rabbit hunting and accidentally shot himself in the foot as he was climbing over a fence.
Like you said, everyone makes mistakes. Unfortunately, when you make a mistake with a firearm, sometimes there are fairly severe consequences.