If you spawn a parrot, and use /effect to give it infinite resistance 5, then feed it a cookie, then even after clearing all of its effects it remains unkillable, including using the /kill command. Arrows and other projectiles will also pass through it like it is some sort of ghost. Using /data get on it shows that its health is NaN. Restarting the server does not remove the parrot nor allow it to be killed again. 

This does not occur in vanilla singleplayer or multiplayer.

To reproduce run the following commands:

/summon parrot

/effect give @n[type=parrot] resistance infinite 5 true

Then feed it a cookie. After this, it becomes undetectable to the @n selector, so you will have to run commands on it using its UUID instead. You can try /killing it or clearing the effects, they wont have any effect.