- Beam Me Up, Scotty!: Without going into spoilers, the game's famous Memetic Mutation is actually "Who else will I have ice cream with?", not "who will I eat ice cream with?"
- Billing Displacement: Mickey's presence on the cover only serves to remind players of the Disney elements in the game, because aside from being playable in Mission Mode, he only appears in one short cutscene and doesn't even meet Roxas.
- Creator Backlash: Tetsuya Nomura profusely apologized for the movie in the I.5 HD ReMIX collection not adapting any of the game's battle scenes, noting that the transition between Xion challenging Roxas and then collapsing in defeat in particular is terribly awkward. The cinematics for the Re:coded movie in the II.5 HD ReMIX collection have short battle cutscenes to remedy this, and the aforementioned scene from the 358/2 Days movie is amended in the PS4 version.
- Dummied Out:
- Profile sprites for Pinocchio characters, Megara, Hades, and Tinker Bell exist in the game files.
- Roxas, Axel, Xion, and Marluxia have slightly different HUD sprites that went unused.
- A model for an enemy called Crooked Chariot is buried in the data.
- There are a couple of Keyblades that have unused palette swaps, such as the Leviathan's counterpart (known as the Wind Maker◊).
- In Memoriam: The English version is dedicated to the late Wayne Allwine (voice of Mickey Mouse), who died just before the game's Japanese release.
- New Work, Recycled Graphics: The FMV cutscenes reuse models from Kingdom Hearts II, and a couple cutscenes are ripped directly from that game.
- The Other Darrin:
- Although Christopher Lee came back to voice DiZ in this game after Corey Burton voiced him in Re:Chain of Memories, he was replaced by Burton again in the 1.5 HD Remix due to added scenes and to keep consistency between the voices.
- The 1.5 HD Remix extended Mickey's sole scene with Riku, but because Wayne Allwine passed away shortly after recording the original, Bret Iwan had to take over. Unlike Lee, whose lines as DiZ were rerecorded by Burton for vocal consistency, Allwines lines as Mickey were retained even in the cutscene movie as indicated by his Posthumous Credit, with Iwan only voicing him in the extended part.
- Swan Song:
- This is not only the last game in the series to feature Wayne Allwine as the voice of Mickey Mouse before Allwine's death in 2009, but Allwines final performance as Mickey in any medium.
- This would also become the final performance of Christopher Lee as DiZ before the actor's death in 2015. From this point on, the character is voiced full-time by Corey Burton.
- What Could Have Been:
- A beta version of the game from 2006 had a MP gauge, suggesting that Kingdom Hearts II's magic system was considered instead of the panel-based one in the final product. There is also a bar labeled "LM", likely a Limit meter that charged up before letting the player use a Limit Break instead of having it be dependent on how much HP the character has left.
- Luxord's Mystery Gear weapon was originally meant to be dollar bills.
- A world based on Pleasure Island from Pinocchio with a What Measure Is a Non-Human?-centered plot was planned, but it was scrapped due to space limitations. Some Pinocchio character sprites are tucked away in the game files, and aspects of this idea were later used in Kingdom Hearts 3D: Dream Drop Distance.
- Dummied Out portrait sprites of Megara and Hades indicate that they were planned to show up in Olympus Coliseum.
- A full remake was considered for Kingdom Hearts HD I.5 Remix, gameplay and everything, but was scrapped due to time concerns.
- Word of God
- Roxas passing out upon entering Castle Oblivion was due to him being too close to Sora and his other self.
- Apparently, Xemnas did something to Xion to make her powerful for when she confronted Roxas the following day.
- According to Nomura Xion didn't always appear as Ventus in Xigbar's eyes. It was just in that moment.
Trivia / Kingdom Hearts: 358/2 Days