The skill Quiver of Arrow: Grade A (don't know about S yet) reckonizes 70 as the inventory maximum rather than 80. I confirmed this for myself while trying to summon arrows a few times over the past few days, none of the cases was it a "true" full inventory with the hidden items you get when picking up but having a laggy SQL. I can summon on 69/80, but not on 70/80 deu to the "full" inventory.
Even when i de-equip my bow and i have the skill's only end-product, A-grade arrows, already in my inventory, it refuses to work.
Greetings,
Asce.