Theres a glitch in Zombies to where if you drink a Speed II before buying Stamina, Speed II (I think its unlimited time) is applied but the timer you had for the buff is still going. Ex: Drink Speed II Potion and have 0:40 left on the timer. Get Stamina-up and the timer is still there and not at **:**
I don't think that can be fixed, point of the lesson, don't get stamin-up while the timer for Speed potions are still up.