Originally Posted by
Slice
You CAN have a bad wire to the mag switch which can cause it to be 'live'. It's best to treat all props like they could be alive at any moment.
Very common with GA recips, you usually notice it when you do the mag check. I'd have to use my toes to count how many times it's happened to me.
Also since the carb bowl still contains fuel, there is absolutely nothing to prevent an engine from running with a broken mag lead. If it's hot and the crank's in the right position, it can start with only a slight bump.
Always, always, always treat a recip prop as a hazard zone.