My buddy wanted to try the trigger pull on my 340 PD, which I was carrying at the time, so I unloaded it, put in snap caps, handed it to him and he squeezed the trigger a few times. When he was done, he handed it back to me, and I took out the snap caps and reloaded it with my carry ammo. He watched me do it.
Then my wife called me, so I set the .357 on my work bench and went to talk to her. A few seconds later, there was a loud "bang" from the garage, and I ran in to find my buddy white as a sheet, staring at a little hole in my wall. He kept apologizing and saying it was his fault, but I take full responsibility... absolutely STUPID to leave a loaded gun on a bench and walk away. But we certainly both learned a lesson from it, and both he and I have been fanatical about safety since then.
In a perfect world, mistakes wouldn't happen, but it obviously isn't perfect. The next best thing is for us to learn from our mistakes and, better still, learn from others' mistakes. This thread is hopefully going to accomplish something positive in that regard.