I cannot tell how, but when playing on one server last month, I've noticed that any Unbreaking enchant may occasionally disappear from a piece of armor. One of my friends has just had Unbreaking III disappear from his pickaxe. Neither server had any plugins that dealt with enchantments, and operator intervention seems highly unlikely. As we don't check our equipment every tick, it is uncertain how in either case that the enchantment disappeared. In either case, I have not yet noticed it occurring with any other enchantments besides Unbreaking. I have noticed that it doesn't happen often.