Negative splash potions do not affect some mobs.
What I expected to happen was...:
Throwing a negative splash potion would hurt zombies and skeletons
What actually happened was...:
They weren't affected by the thrown potion and showed no sign of damage
Steps to Reproduce:
1. Put yourself in creative
2. Spawn a zombie or a skeleton
3. Grab a negative splash potion (Harming, Poison)
4. Throw it at the spawned monster.
You find that only zombies or skeletons go unaffected, it still hurts other mobs.
I've tried it on Spiders, Creepers, Zombies, Skeletons and endermen - all but the zombies and skeletons are affected. Untried on other mobs.
Undead mobs are damaged by splash potions of healing and healed by splash potions of harming. Neither regeneration nor poison have any effect on undead mobs.