Talking of Lyco/Conti engines,
AFAIK the ignition switch has just
one grounding wire coming out of it.
So, the scenario where you get a successful L/R mag test but get a live magneto with the switch in the OFF position, seems exceedingly remote. Is anybody aware of this having ever happened and, if so, any details?
It would need a very strange internal break-up of the contact mechanism, but never having taken one of the switches apart I can't say anything specific.