Partner and I were messing around with the crabs when I had the idea to throw a mob head at it. Turns out it turns into an entity that crashes clients. I have resource packs, but SO got exact same crash while running the vanilla aof pack with Xaero's. So that rules out the resource packs.
#Crash after attempting to equip a Helmit crab with a Slime head
amber drum
fixed with a kill command, but more mob heads on helmit crabs could cause similar crashes