FAQ: Why do I get errors when I set an entity's health?

At time of writing, this is a Bukkit limitation since Bukkit 1.0 - you cannot use MD to set an entity's health above the max or below 0 (the latter for obvious reasons). There will hopefully be a method to override this included soon. It is recommended you use the Heal Calculation in the meantime, which is designed to avoid the IllegalArgumentException that Bukkit throws when you try to exceed the max health.