Thanks for all the answers. However I still haven't found an answer I'm looking for. My worry is that someday when cutting the manetos during the engine run up, the key will get jammed a little in the off position and the engine will stop. I want to know because I'm doing a lot of solo Nav flights now, and if it does happen accidently, I want to understand what happens to the engine, what to do and what to look out for. I've been told that it can be bad for the engine but haven't been really been told why. If its just as Sport_Pilot said, then surely its just a case of starting it straight up again?
So you want yourself or one of your buddies to be chopped up by a runaway aircraft engine?! Buddy if you have a bad ignition switch and start turning off the engine by turning off the engine, its not a possibility that someone will get killed or lose a limb, its only a matter of time! This was a very common occurance in the past!
Do the right thing! Ground that aircraft till the ignition switch is replaced and go through the normal checklist proceedure to turn off that engine. This is a result of age old trail and error safety proceedures that was only found after much loss of life and (literally) limb.