9. The Sea Ring - Lufia II
When the Sea Ring was first designed for Luna II: Rise of the Sinistrals, it appears that it was intended to be of use to the player - at least, at first.
The ring is supposed to afford you extra protection against sea creatures, which could have been incredibly useful, as most games have at least one tough fishy opponent. However, after designing this ring, the devs never designed or marked any of the enemies you face as sea creatures, meaning the Sea Ring is totally without use.
You could say that this was unintentional, but someone on the team definitely noticed that this particular item protected against absolutely nothing, and then decided it was pretty funny if it got left in regardless. Otherwise, the item would have likely been wiped from the game, instead of continuing to exist as a strange mystery that remains unanswered unless you google it.
As it is, you'll likely spend a decent chunk of your play time wondering in the back of your mind what that weird piece of jewellery you picked up is actually good for.