When trying to Combine an enchanted Sword, bow or armor with the same or a different enchantment book it will not work. Its like the game doesn't know how to combine enchanted books with enchanted items.
Reproduce bug by doing these steps:

1. Load/Start a world in Creative or Survival mode.
2. gather 2 enchantment books , an anvil and an item for enchanting.
3. Enchant the item with Enchantment book 1 using the Anvil.
4. Put the sword into the Anvil again and place the second book.

There will be no item at the slot where you usually take out the sword that has been combined with the 2 enchantments. That slot will stay empty instead.

gatharian added:
You're unable to add 2 or more enchantments to a sword for example Sharpness and Bane of Arthropods.