On a modded server, used the Ice Breaker with the Earthquake ability to kill another player but he didn't die.
His hearts are all empty and his screen is shaking constantly, as though he is receiving damage again and again.
He can't die, and using /kill does nothing either.
In his playerdata .dat file, his health is set to "NaN".
He cannot access inventories such as chests or his own inventory, can't pick up items. However he can hurt entities and break blocks.

His client logs feature these errors :

Error executing task on Client 
java.util.concurrent.CompletionException: java.lang.NullPointerException: Cannot invoke "it.hurts.octostudios.octolib.modules.config.impl.OctoConfig.prepareData()" because "config" is null

Failed to process a synchronized task of the payload: octolib:config_sync
java.util.concurrent.CompletionException: java.lang.NullPointerException: Cannot invoke "it.hurts.octostudios.octolib.modules.config.impl.OctoConfig.prepareData()" because "config" is null

Here are the logs: latest.log

Changing both Health and AbsorptionAmount from NaN to 20 or something in the player's playerdata .dat file fixes the issue by the way.
I was able to reproduce the bug by using the ability on him again, and I confirm the exact same thing happened. I was able to fix it again by modifying the playerdata file.
So not too game-breaking of an issue since it's easily fixable, but it does make the Ice Breaker boots unusable.

Fixed