When enchanting an item with enchanted books in the anvil, an unenchanted item can recieve any enchantement, regardless of whether the item can get that enchanment from an enchanting table, or enchantments at all. The enchantment is functional, so fire aspect on an item will still catch mobs on fire when you hit them.

This occurs in both survival and creative mode.

To duplicate, simply place an item in the left side of the anvil,and an enchanted book in the right hand side with an enchantment the first item can't normally have. The item should recieve the enchantment upon retrieving the item from the output slot.

*Edit: It should be noted that items with an enchantment can no longer recieve any enchantment, only recieving enchantments they could get via enchanting table.
Furthermore, this bug does not occur on enchanted books.