Nim Deathmantle's second ability may return a card it can't equip to the battlefield. For example, if a nontoken artifact that's become a creature is put into your graveyard from the battlefield, Nim Deathmantle's second ability triggers. If you pay {4} as it resolves, you'll return that card to the battlefield. However, Nim Deathmantle can't equip it, so Nim Deathmantle remains attached to whatever it was already equipping (or, if it was unattached, it remains so). The same is true if a nontoken creature with protection from artifacts is put into your graveyard from the battlefield, for example.