In Canada it's illegal to leave an airplane running with
nobody at the controls, though we see cropsprayers doing it. The only
exception is for handpropping, and that requires that the airplane be
secured against any possible movement.
Besides that, we regularly hear of folks walking into props,
even with someone at the controls. A fella and his wife were taxiing
out in their Cardinal when she decided she had to pee (sound familiar?)
and so he returned to the ramp and kept the engine running while she
got out and promptly walked thru the prop. End of wife. Is the starting
procedure really THAT much hassle that we can't pull the mixture and
restart a few minutes later??!

Dan