So I've been building up my Villager Trading farms and halls and breeders, etc., and one of these builds is the converter - Using a zombie to convert the villager and back again to reduce trade costs.
I found a Zombie, and it can hold items which (should) mean that it won't de-spawn. I also gave it a helmet so that I could keep it in a boat while I build/if I need to save it from the sun.
However, when I came back to check on it, the zombie was still in the boat, still holding the item, but the helm had gone, and he burned in the sun.
What happened?