Some enchants, I'm using a diamond chestplate for example, with full protection, full thorns, and full unbreaking,

can be applied with a book, which is at the maximum level possible and it will still apply to the armour piece, instead of giving a "too expensive" result, despite making absolutely no change to the armour piece, except for having the price of the armour piece being upgraded shot up substantially.

Reproduction Steps

1. Put a maxed out armour piece in an anvil

2. Apply another maxed out enchantment it already has.