A further point about the dangers of a live prop.
After shutting down our gliding club's C-182, we always put the keys on top of the glare shield so that someone outside can see them.
However, that is not a guarantee that the engine can't start. Apart from the obvious issue of a potentially broken ground wire, it is also possible with some keys, to remove them from the ignition switch, when it is set to Both.
I have personally had that happen to me with a PA-28 and I've heard that it is fairly common.