Some of my players came across this bug earlier today and it can be a real inconvenience. In order to replicate, I follow these steps (Running on Spigot #807) 1. Enter the server and be in survival mode. 2. Obtain an anvil, diamond shovel, Unbrekaing 3 enchanting book and 5 levels. 3. Enter the anvil GUI and place shovel first, book second, client says the action would requite 5 levels. 4. Take your object out of the processed end of the anvil, move it around in your inventory twice. 5. You will now have the action cancelled, items moved back into the anvil but client side your levels will be removed and require a world change or re connection to regain them. Bug was reported nearly a month ago as BUKKIT-3863 but appears to of been ignored.