Affects Version/s: Minecraft 15w31b, Minecraft 15w31c, Minecraft 15w32a
Fix Version/s: Minecraft 15w32b
UPDATE: As of 15w32a this bug only exists for Armor Stands. All other mobs and players appear to work.
Any item can still be put on a player's head via replaceitem, but mobs can only wear helmets. This appears unintentional as per https://twitter.com/SeargeDP/status/623125935445921792
/replaceitem entity @e[type=ArmorStand] slot.armor.head minecraft:diamond_helmet
Does not work:
/replaceitem entity @e[type=ArmorStand] slot.armor.head minecraft:apple