Spirit Reservation.

If you equip a weapon / skill gem for a skill that reserves spirit, (e.g. skeletal warrior minion), if you add minions/toggle aura so that the skill begins to reserve spirit - then you subsequently unequip the weapon/skill gem without going in and reducing the minion count to zero - the spirit will continue to be reserved without any benefit.

Also, I have Enfeeble socketed in Blasphemy. When I attempt to toggle the aura, it shows that it costs zero spirit. Whether it is toggled on or off, it has no effect. In weapon set 1 if I toggle it on, it does not change the amount of spirit that I have remaining to use. In weapons set 2 if I toggle it on, despite saying zero cost, it will reserve 59 spirit, and still has no effect.