If you defeat Maleficent in Hollow Bastion before chasing Pinocchio in Monstro, a set of alternate cutscenes will play out as you progress through the rest of Monstro. The cutscenes where Riku talks to Sora in the first chamber of Monstro, and the cutscene where he talks to Maleficent are both edited out of this sequence.

A Station of Awakening for Alice from Wonderland was originally created and planned to appear in this game and accompany the Belle, Cinderella, Aurora, and Snow-White ones but ended up never being used. Spoiler:With a possible explanation being the fact that Alice, like Kairi and Jasmine and unlike the other Princesses of Heart, was not abducted by the Disney villains at the very start of the game.

Early screenshots of the game showed Sora with different clothes that were orange and black and a different hairstyle that was black but with large white tufts sticking out the front of it. This early design for the character also resembles the earlier "Lion Sora" design, albeit without the lion elements.

Early concept art shows that Sora would have originally donned Aladdin-inspired shoes and a more street rat-inspired version of his usual wear when he was in Agrabah, but in the released version he just wears his normal outfit instead.

In early concept art for the game, there is a world with a heavy Wild West aesthetic that is nowhere in the final release. It is unknown what Disney IP, if at all, that it would have been based on.

When coming up with a name for the game and eventual series, the developers were attracted to the name "Kingdom" because, to Nomura and his team, the game's iconic premise of the different Disney movies' stories being rendered as "Worlds" for the player to visit reminded them of Disney's theme parks, such as the Magic Kingdom and Disney's Animal Kingdom, which are segmented into different "lands" or areas for visitors to explore. However, Square had difficulty in securing the name "Kingdom" alone, so they decided to add the word "Hearts" to it because such entities played an important role in the lore of the game and series.

Disney heavily restricted the use of Mickey Mouse in the game especially as the central character like Square initially wanted. This was because Disney was nervous about the success of the game and did not want its possible failure leaving a black mark on the company and its mascot. Tetsuya Nomura stated that these contractual restrictions went so far as to have Disney allow Mickey to only make a minor cameo appearance (such as "in the background of a crowd" as Nomura put it). Spoiler:Nomura and his team got around this by having Mickey have an extremely important but brief role at the end of the game in the form of Mickey helping Sora close the Door to Darkness so no more Darkness gets out of it, separating the two. King Mickey even appears mostly in silhouette in this scene. When the first Kingdom Hearts became a humongous success for both Disney and Square, the former allowed the game developers to feature their mascot in a much more prominent role in the franchise from then onward.

According to series composer Yoko Shimomura in a KH Ultimania interview with the game's sound team, they believed the operatic song "Destati" was "cursed":

In Huey, Dewey, and Louie's Item Shop in Traverse Town, there is a poster of a steamboat with the caption "RIVER BOAT" in a large cartoony font, possibly as a reference to the 1928 Mickey Mouse short film Steamboat Willie. The art style of the late 1920's black and white Disney cartoons as a whole would be adapted as an explorable area in Kingdom Hearts II called Timeless River.

Sora, Kairi and Riku's ages at the chronological time of this game's story are all stated in the PlayStation 2 release's instruction manual. Specifically, Sora and Kairi are both 14, and Riku is 15.

Kingdom Hearts' international release is the last game by Squaresoft released outside of Japan to have their logo and name adorned on its case before the company merged with Enix to become Square Enix in 2003. The Japanese version of Final Fantasy X-2 was last game ever to feature the Squaresoft logo in any region.

Concept art featured in early design documents of the first Kingdom Hearts game shows a world clearly based on Disney-Pixar's Toy Story franchise that, for reasons that have not officially been stated, does not appear in the final release. A Toy Story world would, however, appear in Kingdom Hearts III

During "Dissidia Final Fantasy NT: Behind the English voice cast" with Steve Burton (Cloud Strife's English voice actor), Steve explained the reason why he got the role for Cloud, was because of a guy from "Square" who's in charged for the voices of the characters, was watching a movie of him in "The Last Castle".

A number of Gummi Block Blueprints use the names of characters and enemies from the Final Fantasy series.

During a Q&A session at MomoCon 2015 with the creators of the popular cartoon series Gargoyles, they revealed that the show was considered as one of the worlds in Kingdom Hearts. However, some unfortunate circumstances caused the inclusion to be halted after some team members left Disney.

Original Kingdom Hearts design documents reveal many differences between early game concepts to those that appear in the final game. These include the appearance of a strange icon depicting a drastically different Destiny Islands, unused Heartless designs, and Sora having clothes similar to Tarzan in Deep Jungle.

Kathryn Beaumont, the original voice of Alice in Alice in Wonderland and Wendy in Peter Pan, reprised her role as Alice and Wendy in Kingdom Hearts at the age of 64.

A red Trinity can be found in Halloween Town that must be collected before Spoiler:Oogie Boogie is defeated, otherwise it will become impossible to collect and the player will be unable to get a 100% completion rating in Jiminy's Journal. The red trinity is located Spoiler:inside Oogie's Manor, which disappears when Oogie is defeated. This was later fixed on the Final Mix release of the game by relocating the Trinity to the ground.

After the release of the first Kingdom Hearts game, Disney was planning on making an animated series based on the game. A full-color, voice-acted animatic pilot for the show was produced and received well, but ultimately the show was never made. Since Kingdom Hearts: Chain of Memories and Kingdom Hearts II were formally announced around the time the pilot was made, Disney opted for Square-Enix to continue the story with more games rather than through an in-house animated series.

In 2022, the animatic pilot was uploaded online by its director Seth Kearsley.

In Kingdom Hearts Final Mix's secrect ending, Another Side, Another Story [deep dive], Mickey can be seen wielding Sora's keyblade, the Kingdom Key, and Roxas holding Mickey's keyblade, the Kingdom Key D. These concepts were, along others (such as Roxas having yellow eyes), not used in any of the later games and they only served as a preview of what Tetsuya Numora had planned for the future of the series.