Minecraft wiki entry for Development_versions, associated text: MC-68565 – Monsters spawn at daytime at y=256 and don't burn.
MC-72809 – Entities do not have shadows at y=256.
MC-111480 – Firework boosting doesn't work in the first world loaded after a game start or if player has entity id 0.
MC-125051 – Incorrect structure names in advancements cause java.lang.NullPointerException
MC-139712 – Copying one half of unrecognized Unicode character crashes client.