There are four unused items in the game's memory obtainable only through GameShark codes or debug rooms:

Masamune: A Long Sword-class Artifact weapon, increases base attack by 115 and has no unleash effect. It has a unique graphic that is not seen legitimately in either this game or Lost Age.
Sol Blade: A Long Sword-class Artifact weapon, increases base attack by 138 and has no unleash effect. Its 138 attack power is the highest out of any weapon in this game, surpassing even the 135 attack power of the Gaia Blade, this game's ultimate weapon.
Kusanagi: A Light Blade-class Artifact weapon, increases base attack by 135 and has no unleash effect.
Mysterious Robe: An Artifact robe, increases base defense by 40 and has no secondary effect.

Most of these items became proper, fully-coded items in the game's sequel, Golden Sun: The Lost Age
Contributed by MeleeWaluigi
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In the first Golden Sun game, you can get a "bad ending" near the beginning of the game. After the events that occur at Sol Sanctum, the Great Healer will ask you, "Will you accept responsibility for the fate of the land?" If you say no and leave the room, the screen will turn grey and display the text "And so the world began drifting toward its fated destruction..."
Contributed by DidYouKnowGaming