Not (quite) so fast
Until recently I would've agreed than it SHOULD be impossible to have an "accident" while cleaning a firearm. BUT...
A month or so ago I went to the range and used my Marlin Model 60 (tube-fed, semi-auto, .22). I generally load/shoot 10 rounds at a time. Don't count shots, as bolt locks back when empty. I finished my last set of 10, and placed the gun in the case.
When I got home I started cleaning the gun. I can't tell you the exact sequence, but it was something like this: Run a patch or 2 w/ Hoppes down the barrel, then dry patch then light oil. Release bolt. Clean bolt and bolt face(requires opening an closing bolt). Remove, wipe down, lubricate and replace magazine tube. Lubricate, lightly, the action, including pulling the trigger AT LEAST ONCE, to allow trigger to move so I can put a drop of oil in there.
In short I worked the action at least 3 times, and dry fired it twice, somewhere along the way. And when I was all done, I opened the action once again, and heard a sickening noise as a LIVE round fell onto the floor at my feet. I have NO idea where that was "hiding" inside the gun (suspect the magazine tube, somehow), and I'm STILL trying to figure it out.
Now, the one thing I did, as always, was keep the muzzle pointed away from anyone (Actually I was alone. But, of the the 4 rules, this should be the most important, because if all else fails, at least it minimizes the consequences). So, had the gun discharged, I probably might've shot my laptop, as the gun pointed in that general direction.
It may sound stupid, but this was very scary for me, as I'm downright PARANOID about safe handling of firearms.I've beaten the 4 rules into my kids heads for years, and I'm proud of how careful and responsible they are. And, obviously, the guy in this case didn't do that.