If an item has an invalid enchantment ID (between 0-255, but not one that is in use), the item will cause the client to crash when placed into an anvil.

Any other actions involving the enchantment (such as using the item, or hovering over it) do not cause a crash, as the validity of the enchantment is checked ('if Enchantment != null'). This sanity check is not performed in the anvil GUI.

Steps to reproduce:

1. Put any kind of item in your inventory
2. Use an NBT editor to put an enchantment on the item using an ID not used by any existing enchantment (i.e 200)
3. Go back ingame, the item will have an enchanted glow, but not list any enchantments (as expected)
4. Place the item in an anvil
5. The game crashes