Mangos Version: 9599
How it SHOULD work: When you use an ability like Envenom or Feint that applies a buff, you should always get the buff if the ability didn't miss.
How it DOES work: If for example you hit with envenom and it doesn't miss, and you have high avoidance, you may dodge the buff applied to you that increases chance to apply poisons, in the combat log it reads something like : "Player's Envenom was dodged by Player"
It's an annoying bug that hurts mutilate dps, and it may affect other spells, but I only noticed this bug for Envenom and Feint.