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Final Fantasy Wiki - List of permanently missable items
-The elemental rods that can be bought on Bartz's World can be lost if the player does not buy them before moving onto Galuf's World. The other items that can be bought only on Bartz's world is the Iron Shield and Kenpo-Gi in the town of Walse.
-The ice summon Shiva must be fought and defeated to become usable before the mid-game plot twist, or she becomes unavailable. She is located in the water tower in the back of Walse Castle.
-The song Romeo's Ballad and Alluring Air can be learned in Istory and Lix on Bartz's World only.
-All the Genji Equipment can be stolen from Gilgamesh each time the players encounter him.
-The random boss Catoblepas must be fought and defeated in Galuf's World to become usable, or it becomes unavailable by the Merged World.
-The Power Staff can be earned from the Black Warlock in Castle Exdeath only.
-The summon Carbuncle must be fought and defeated in Galuf's World to become usable, or it becomes unavailable by the Merged World.
-The Excalipoor sword will be missed if the players ignores its treasure chest in Castle Exdeath or kills Gilgamesh before he transforms into his final form.
-The song Mana's Paean must be learned from the Library of the Ancients's terrace in the Merged World before the battle against Melusine in the Guardian Tree.
-The Thief's Gloves can only be stolen from Sekhmet in the Pyramid of Moore or Necrophobe in the Interdimensional Rift. If the players kill Sekhmet, he'll never appear again.
-Either the Brave Blade or Chicken Knife will be missed by choosing the other choice.
-The accessory Kornago Gourd is a reward for the sidequest in Quelb and can be stolen from Omniscient in the Fork Tower.
-Gold Hairpin can be stolen from Famed Mimic Gogo in the Sunken Walse Tower.
-The Titan's Gloves can be stolen from Azulmagia in the Interdimensional Rift.
-The Titan's Axe can be stolen from Twintania in the Castle in the Interdimensional Rift. The Tinklebell is also earned from Twintania.