Well, the one case was after mag work was done...mechanics completely forgot to attach one P-lead. The other time, the connecter (at the mag) just wore out and broke.
I've had the switches themselves do funky things too, but only one that I could pull the key out from with it on. The problem I always had was they moved too easily and the "click" was too slight, so students would go from both to off during the runup (BANG....•!!!!!), or hit the key with their knee in flight and unknowingly turn it to the left mag only (gee, why are we not climbing so well after that touch&go?!?!). There is an A.D. on most of those mag switches, but its just for the wiring, not for the switch itself.