Issue seems to only be replicable on a server when it has low TPS.
Happened on a Fantasy MC Fabric server, while it was in v2.5.0, with Mine Cells v1.7.1, latest release on Curseforge.

Issue / Exploit:
You are immortal. Can no longer die to anything at all, including /kill, the curse itself when proc'd again, you can no longer die in any way.
Side-effect, your screen is violently shaking forever, until issue is fixed by resetting your Player Data.
Even changing Distortion Effects and FOV Effects down to 0%, your screen continues shaking.
Exploit is continuous between relogs, as it corrupts your Player Data.

Reproduction Steps:

• Play a Fantasy MC Fabric server, while its TPS is < 20
• Hold Cursed Sword
• Swap off of Cursed Sword
• Once unequipped, immediately after the Curse expires, get hit by anything to die from Curse
  ( You no longer have the curse on your client-side, but the server still thinks you do because of lowered TPS )

Result:

• You now have NaN health, using /health query [affected player] , displays this corrupt player data
• Your screen is violently shaking

Attempted solution steps:

• /health set [affected player] [any valid number]
• /minecells:clear_data [affected player]
• /kill [affected player]
• Have player hold sword and die to the curse again without swapping off of it
• Have player drop into the void for an extended period of time
• /execute as [affected player] run gamemode spectator , then, /kill [affected player]
• /execute as [affected player] run gamemode creative , then, /kill [affected player]

Solution:
Since their player data is corrupt, the only solution seems to be to reset that data.

Conclusion:
If the issue goes un-reported, it could be easily exploited for immortality forever.
A resource pack or mod to disable all screen effects could be used to get past the side-effect.

Thank you for the expansive report! Which version of Mine Cells was used in that version of the pack? I made some changes to the Cursed Sword recently, so I'd like to know if the issue occured before or after them 😅

Of course! The pack has Mine Cells v1.7.1, the latest release on Curseforge.
Pack includes Mine Cells v1.7.1 in v2.5.0 of the pack (where the error occurred), and did not change that for the pack update to v2.6.1-HF2 (latest release of the pack),
Sorry, forgot to include that.

Okay - so that's happening after the changes... which were made because of another invulnerability exploit 😁
I don't really have a way to test this bug or verify if it's been fixed. Is it okay if I make gradual changes that might work and release new versions with those, and rely on you to try and replicate the bug?

Sounds like a plan.
I can sort of reliably re-create the issue w/ a Local Private Server, and a Lag Machine in Force Loaded chunks to simulate lowered TPS.
I'll make sure to check to see if its fixed in updates to come :)

Hey, the first update that might have changed something about the sword is out! 😁

Awesome! What you changed seems to have worked!

After ~20m of testing, I can't seem to reproduce it at all on the new version.
Usually in the previous version, when the issue happens.. You see the Death screen for a few frames, then you get hit by the Curse after the death screen, wake back up, and are immortal with your Player Data corrupt.
Now, when this happens (You are hit by the Curse after the Curse already killed you and expired on your client-side), you don't wake back up at all. The Death screen just stays up and you are able to respawn like normal w/ no data corruption.

Thanks for the quick responses and repair of the issue!

• Note: Like I stated previously... my reproduction methods im not sure are Entirely reliable as the window to reproduce it is extremely specific.. so yea