there is a bug that causes a invincible dying mob to be created.

description:

• red
• has a die/heal die/heal animation
• seemingly undefeatable
• haven't seen if it can survive /kill
• only way to get rid of the mob is to switch it to peaceful or wait for it to despawn

how to make a invincible dying mob:

• use a wither potion to kill the mob
• just when it is killed, throw a healing potion on.
• now you have a invincible mob

picture:

19 Survival Hacks to Save Your Life in Minecraft - YouTube

go to number 14 on the link for more info