If a witch a accidently threw a poison,weakness or slowness splash potion at a skeleton or stray,it will start shooting at the witch,and the fight will never end because witch will keep throwing poison,harming and slowness splash potions at the skeleton and regaining its health
In addition that undead mobs are immune to poison and can be healed by harming splash potion,so witches should throw decay splash potions at undead mobs to hurt them.