Know thy systems. The P leads on the mags ground out when the switch is in the off position. If a P lead has an open (break or bad connection) the mag will be hot. That is why one always does a quick mag check by turning switch to off for a quick instant before shutdown - to catch it before the condition can go on for long.
Here is yet another golden recommendation I want to make sure I implement moving forward. I have already seen a too close call for comfort event, I wish never to see another.