Always assume a weapon is loaded if you haven’t checked the chamber.
Always assume a weapon is loaded. Period. Even if you've checked the chamber. You can be mistaken on what you see, especially if you're tired or not paying attention.
When I was in Iraq one of the guys in my unit had a negligent discharge. He swore he checked the chamber for a round. My guess is he halfway racked the slide and thought he saw the side of the chamber when he really saw dirty brass. Luckily he fired into the clearing barrel but you don't want an ND ever.
Your eyes and brain can lie to you. Don't trust them.
I think the point here is that anyone without training can pick up someone else's firearm and fire it believing that it's empty when it's not. A child wouldn't necessarily know that there's still a bullet in the chamber.
People sometimes forget where they left things. Even important things. Even things where somebody can die if you don't remember in time.
It's kind of like asking "what kind of accident are you imagining where someone leaves a baby in a hot car?" Nobody is expecting that to happen to them, but sometimes a person just forgets for no logical reason.
That's not to say they're not still responsible for whatever happens, though. But it's still good to give consideration to the possibility.
I realize any sort of regulation whatsoever is anathema to a lot of people who own guns, but I really think we would cut down on gun deaths by a huge margin if you were required to take a gun safety class before you could buy a gun. I'm sure you could give a basic overview of gun safety with one short class and it would be enough to stop a great many accidental deaths.