A broken magneto P-lead can be very dangerous. One or both magnetos incorporate an impulse coupling as a starting aid. Moving the propeller just a few degrees may be enough to trip a cocked impulse coupling, and without the electrical grounding provided by the P-lead, a spark will be generated in a cylinder. If it ignites residual fuel, the prop may swing with considerable force. Anyone standing in the prop arc will earn a trip to the hospital, or worse.
It’s common to find P-leads hanging from their crimped terminals, so it’s not a surprise when engine vibration or accidental abuse disconnects either the shield or the center conductor. The wire needs support, and it’s easy to do. Slick mags ground the shield with a case screw that can be made to do double duty. When assembling the wire and crimped terminals, add a short length of adhesive heat shrink for protection, and fixate the wire to the ground screw with a plain steel Adel clamp.