If a mob is splashed with Healing and Harming at exactly the same time while on 1 or 2 Health, they will become immune to all damage except potions. This can be done using dispensers. In 1.18.30 or above, you cannot even hit the mob. On previous versions, you could hit the mob but it would deal no damage. This could be used as an exploit to kill players with a single golem or to cheese the Warden with golems (after 1.19).