When using instant damage potions on mobs, when they are supposed to die, they make the noise and give their drops and xp, but the mob continues to walk around. You can then use another potion on it to 'kill' it again and get more loot, but the mob continues to walk around. I have tried doing this in creative mode for every mob by throwing Splash Potion of Instant Damage II on each type of mob. All horse variations, blazes, and undead mobs that must be killed with instant health instead, as well as witches are unaffected by this bug. I was unable to test with endermen, and creepers seem a bit broken, they didn't drop anything, or make noises when they took damage.