You are unable to kill the wither if you use /kill @e[type=WitherBoss] while the wither is invulnerable. It will say "Killed Wither", but will have no effect.
Environment
Windows 8, Java 7
You are unable to kill the wither if you use /kill @e[type=WitherBoss] while the wither is invulnerable. It will say "Killed Wither", but will have no effect.
Windows 8, Java 7
Confirmed
Minecraft 14w02c, Minecraft 14w06b, Minecraft 14w08a
Minecraft 14w10a
Retrieved
Confirmed in 14w04b.
Dinnerbone has indicated that /kill should work even on invulnerable entities, so this is definitely a bug.