If you apply an enchantment outside the normal minecraft range (e.g. /give @p diamond_sword 1 0 {ench:[

]} ) it will create an enchanted item that has no usable enchantments. This can be useful to stop someone from being able to enchant it again. However if you place this item into an anvil and try to add another enchantment via a book it will crash the game. Only when the enchanted book is placed into the anvil will the crash occur.