CosmeticArmor-1.18.2-1.3.1
twilightforest-fabric-1.18.2-4.1.238

When a player has on Twilight Forest armour, then tries to put any cosmetic armour over top of it, the game crashes. This was reported on Twilight Forest's github first, but they said it is an issue with this mod

How to reproduce:

1. Put on any twilight forest armour. In this case I was using Fiery Leggings and Boots, and Phantom Chestplate and Helmet.
2. Use any other armour as a cosmetic. In this case I used Chain Boots (just since they're cheap), but other players have reported it happening with other mods also.

Link from TF's git: https://github.com/TeamTwilight/twilightforest-fabric/issues/74
Crash Log: crash-2022-08-11_18.15.49-client.txt

Can reproduce the crash reliably in the modpack if all 4 armor slots and all 4 cosmetic armor slots are filled with Twilight Forest and Bumblezone armor together. Then got it again if using 7 Bumblezone armor and 1 vanilla armor. But could not reproduce it with random mix of a few other armors or even just vanilla armor alone. No idea what the specific trigger is. Only happens in the Medieval minecraft modpack. Using just this mod, Twilight Forest, and Bumblezone alone does not produce the crash with same armor setup.

Got two more stacktraces. One is crashing on Immersive Armors and another on an unknown mod's mixin. One crashed due to null item stack for what is equipped in the slot and the other crashed while trying to get the item in the slot with getEquippedStack. The crash seems to specifically happen upon adding the second armor but rarely and difficult to reproduce reliably.
https://paste.ee/p/FqPWe
https://paste.ee/p/UHxkB

Somehow, Cosmetic Armors or another mod is causing issues with what the slots returns or has in itself.
The mixin modified mc class of ArmorFeatureRenderer for debugging purposes: https://paste.ee/p/D2VDi

World with my user crashing while having a mix of Twilight Forest armor and Bumblezone armor (filling all 4 armor slots and all 4 cosmetic armor slots)
crashingSinglePlayerUserWorld.zip