enchanting a item with a enchantment you should not be able the item with

What I expected to happen was...:
the enchantment should not be able tom be on the item

What actually happened was...:
the anvil didn't stop me from enchanting the item

Steps to Reproduce:
1. take an anvil, a pice of armor, tools or weapons and an enchantment.
2. put the item and the enchantment inn the anvil.
3. take the item on the right out and it should have the enchantment on it.