Final Fantasy IV/Cheats and Secrets

Extra Items
On the U.S. SNES version entitled Final Fantasy II, the items held in a character's left and right hands can be duplicated. When in battle, select the Item option. Move the cursor down to an empty slot in the item bag, and select the empty slot. Then, move the cursor all the way up until the character's equipped items appear, and select one; it is important to select an empty slot from the item bag before selecting the character's equipment. The equipped item will then be switched with the empty slot. (Note: Both of the character's equipped items can be unequipped this way, one after the other, and they can both be duplicated as a result.) Next, finish the battle. After the battle is over, access the menu screen and go to the Equip menu. When the equipment is given back to the character, the numeral 2 appears on the right. Immediately remove the equipment, and there will be two of them in the item bag. The character can have their equipment back, and the duplicated item will now stay in the item bag. This trick is especially useful for duplicating rare or expensive items. Valuable items can be sold to shops, or simply given to another character who can equip it. Also, duplicating powerful weapons can be useful for Edge's throwing ability.

Sylph Bug?
Also in the U.S. SNES Final Fantasy II, Rydia's Sylph summon (obtained from the Sylph Cave sidequest) does not use any MP. This spell damages all enemies, and simultaneously heals all allies.