I was sitting in a champ (no electrics) getting ready for a hand start for some ski flying. We were at the priming stage where the flipper yells "switches off" and pulls it through - even more important in winter. The mag switch (old lever type) is behind the front seat and the sun was low and in my eyes. I did not have the switch all the way off - I thought it was, but I only fly that plane once or twice a year and I got it wrong - one mag was live. The flipper was demonstrating hand propping to another guy, who was quite nervous. He said, as he turned it through, "always assume it is live, even if the switches are o...." the words weren't out of his mouth when it fired and started. He looked at me like "you @#$%@#$%". And then, in a panic I did the dumbest thing ever - I shut it off! I have no idea why. I should have kept it running because that was the whole objective of the exercise and also I would not have been able to hear him cussing me out. I tried to pass it off as just giving the new guy a demonstration of why to treat the prop as live, but that did not get me very far. I paid for that for many weeks and rightly so. I screwed the pooch on that one and it could have been deadly.
At least when hand propping, people are aware of what is going on and are usually trained and the airplane is chocked, or there is a guy in the cockpit. Even then it has sometimes gone wrong. But when doing maintenance or pre-flight, people are totally not expecting ignition and the airplane is not secure, there could be stuff or people in front of it and what follows is usually really really bad. So be careful out there. Mag check at shutdown certainly can't hurt.
