Contest Winner Cameo: Kurt Zisa, a Heartless Bonus Boss present in the international and Final Mix versions, is named after the winner of the contest held in the USA two months prior to the game's North American release. The boss appears if you try to fly to the Cave of Wonders from Aladdin's house after visiting Hollow Bastion.
Cowboy BeBop at His Computer: Dan Birley, the author of the game's official strategy guide, stated that Sephiroth shouts an attack called "Sin Harvest". In actuality, the line is supposed to be "Descend, Heartless Angel" but the echoing voice and loud sound effects make it easy to mishear it.
Kathryn Beaumont, who voiced Alice and Wendy in Alice in Wonderland and Peter Pan, respectively, reprised her roles despite being in her mid 60s at the time. She's also one of only a few actors/actresses who worked with Walt Disney directly to be involved with the Kingdom Hearts games as well; John Fiedler, who voiced Piglet, did the role in the games as well until his death.
Jodi Benson, who was 41 at the time, once again voiced the 16-year-old Ariel.
Rikku from Final Fantasy X was going to make an appearance as part of the Final Fantasy gang in Traverse Town, but she was replaced with Yuffie. As a nod to this, Yuffie wears an outfit originally designed for the Kingdom Hearts version of Rikku.
Squall from Final Fantasy VIII was originally designed with longer hair - and Leon's appearance here is a reference of that.
Fandom Nod: Aurora's dress is colored purple on her illustration during the Awakening. This is a nod to the Sleeping Beauty fandom sometimes offering purple as a compromise to the pink and blue color war the fairies have.
Kevin Michael Richardson replaced Samuel E. Wright as the voice of Sebastian. This was the first time someone other than Wright voiced the character.
Due to the characters being much younger than their canon counterparts, Tidus and Wakka have different voice actors. Tidus is voiced by Sean Fleming instead of James Arnold Taylornote and Cree Summer as a kid, making Fleming a double Other Darrin, while Wakka is voiced by Dee Bradley Baker instead of John DiMaggio.
The German voice of Hercules is an interesting case. In the movie, he was voiced by Til Schweiger, while in Kingdom Hearts, he's voiced by Dominik Auer...who actually voiced young Hercules in both the movie and the TV show.
Limited Special Collector's Ultimate Edition: Several months after the game was released in Japan, a Final Mix version was made with extra content and a new secret ending. This version later got included in the 1.5 ReMix.
Disney Castle was removed as a playable world relatively late in development but its icon still appears on the world select map, unable to be interacted with by the player.
Early trailers suggest that Donald and Goofy were supposed to wear their classic outfits from the cartoons before they were given new, more fantastical clothing designs. In the final scene of the game, for no reason given, the two appear in their classic outfits which they hadn't worn at any prior point in the game.
Screwed by the Lawyers: The reason Mickey doesn't have a major role in the game is because he was being used for Capcom's Disney's Magical Mirror Starring Mickey Mouse, and Capcom wasn't willing to share the character rights with Square Enix. After heavy negotiation, Capcom agreed to allow Mickey in one scene as long he was in silhouette.
Sleeper Hit: The game was this for Squaresoft following the very polarizing reception it received when it was first announced.
Talking to Himself: Barring a ton of examples in the English version; in the Japanese version, Donald Duck and Genie are both voiced by Kōichi Yamadera, along with Sebastian, Beast and Mushu.
Urban Legend of Zelda: It's rumored that Ariel was going to be a Princess of Heart originally, but they couldn't find a way to work her getting legs and captured by Maleficent after defeating Ursula - and thus was hastily replaced with Alice at the last minute. However, given that she is that world's party member, this is unlikely to have been the case.
The original concept for the game had Mickey as the main character, with Donald and Goofy helping out. The development team eventually became divided over whether Mickey or Donald should be the true main character. As a result, they compromised by creating Sora.
The original scenario involved Sora saving the Princesses of Heart with the assistance of their respective princes.
Sora's original design was more lion-like and wielded a chainsaw sword. This was changed to a giant key to be less violent.
Many early screenshots for the game showed Donald and Goofy wearing their classic outfits from the original Disney shorts. This was changed to the current attire they wear in the game, presumably to give them more adventurous appearances, and they can only be seen wearing their classic attire in the game's epilogue.
A sequence from Kingdom Hearts III, The Final World, was originally meant to be portrayed in this game, but it was changed to Sora turning into a Heartless.
Kairi's role in Hollow Bastion was going to be larger due to Sora going to the Final World. Additionally, it had been toyed around with to make her playable at some point within the game, but that never materialized until years later with Kingdom Hearts III Re:Mind.
Bahamut, a recurring Final Fantasy character usually included as a Summon, was originally going to be featured. While it is possible to hack the game to include Bahamut in the Summons menu, choosing him will crash the game.
There was planned to be one more battle against Ansem-possessed Riku at the End of the World. It was very nearly completed, but Dummied Out in the final version. It would have taken place on a crumbling island - which would have been a remnant of the Destiny Islands, consumed by darkness.
Before Ansem was created, Chernabog was going to be the final boss, being the embodiment of all darkness. This was changed for various reasons. Some concept art shows that the battle would take place at Hollow Bastion as he would have emerged from the Final Keyhole (End of the World was not created until after Ansem was.)
Some concept art showed Sora wearing a loincloth in the Deep Jungle and a world icon for a Toy Story world.
A world based on The Lion King (1994) was planned, but Square did not have the technology necessary to make Sora's lion model function properly. Instead, Simba only appears as a summon and the world was eventually featured in Kingdom Hearts II.
Disney Castle was going to be a playable world and was shown in the game's beta trailer. Although it was removed, it can be accessed with a cheat code (since it still appears in cutscenes).
Aerith was originally going to have her name spelled the same way it was in the parent game. But as it was changed late in development, you never hear the other characters say it aloud. Her name is not actually spoken aloud in the Compilation until Crisis Core.
Aerith's role in the game was originally going to be filled by Aya from Parasite Eve until staff who'd worked on Final Fantasy VII requested it be Aerith instead.
Nomura planned for Irvine from Final Fantasy VIII to be part of Destiny Island's Final Fantasy trio but couldn't come up with a design to fit the island theme, so he went with Wakka instead.
Tifa from Final Fantasy VII was planned as an optional boss, but had to be cut for time. She later appears in II.
"Destati" is apparently considered cursed by the production team due to odd incidents—the worst being the entire building losing power—occurring whenever the vocal track was inserted into a piece of background music.
For the remastered version of the Final Mix re-releases, Nomura himself stated that the team had to rewrite the whole code of the game from what was available from the original game since the original version's game data was somehow lost, which lead to a ton of research, and stated that "it wasn't easy trying to recreate everything in HD from there".