Whenever you wear a thorns enchanted armour, rather than mobs taking damage when they hit you, you take damage when you hit them.

What I expected to happen was that whenever mobs hit me, they will take the damage. This is how the Thorns Enchantment should work.

What actually happened was that, whenever I hit a skeleton, zombie, spider or an enderman, I take the damage. The higher thorns level it is, the more it happens.

Steps to Reproduce:
1... Enchant your armour to thorns (Thorns III is best to see the full effect)
2. ... Spawn some zombies or skeletons and hit them a few times, then you will take damage.