According to series producer Tetsuya Nomura, one reason that Mickey Mouse's role was limited in the original Kingdom Hearts was because around the same time as its release, Capcom owned the license to use Mickey Mouse for the game Disney's Magical Mirror Starring Mickey Mouse. The developers persisted and were eventually given the go-ahead to add him into the game "as long as [they only had him in] one scene, from far away, as a silhouette, with him waving his hand or something". As such, Mickey only appears briefly and seen from behind near the end of the game, helping Sora and Riku seal Kingdom Hearts itself.

Mickey's perspective of this event also appears in Kingdom Hearts 0.2 Birth by Sleep: A fragmentary passage. The reason that Mickey loses his shirt is to partially keep the continuity of the original Kingdom Hearts that showed Mickey in his classic shirtless look.

In the Bizarre Room in Wonderland, the fireplace's flames can be "put out" if Sora attacks the fireplace's grated opening with a Blizzard spell, and it can be lit anew if the grated opening is hit with Fire magic instead. Being able to put out the fire makes the second phase of the Trickmaster Heartless boss fight even easier, as the frantic boss cannot light their juggling apparatuses and fling their own Fire at Sora. This is also possibly why the Cheshire Cat gifts Sora a Blizzard spell/upgrade when Sora obtains and shows him at least the Claw Marks piece of evidence during Alice's trail, and also why the grated opening itself is able to be locked onto.

In each Dive to the Heart segment in almost every game in the series, there is a disembodied voice that can only speak through text that guides the characters (and thus the player as well) through the tutorial of each game. The identity of these voices has been unidentified in every game but Kingdom Hearts: Birth by Sleep (where it is obviously child Sora) and the first Kingdom Hearts. For the latter, in the Kingdom Hearts Ultimania, Nomura was asked who the voice is for the opening of the first game and he finally gave it's identity as being none other than King Mickey. However, this interview was done in the early 2000's before most of the sequels were produced, so it's unknown if this explanation was retconned or not.

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.

Outside Merlin's House in Traverse Town in the first Kingdom Hearts game, the likeness of an owl can be found on the broken archway in front of the house as well as in the form of several small statues surrounding the building. Outside Merlin's House in Hollow Bastion/Radiant Garden in subsequent games, this owl's presence is reduced to two emblems hanging over Merlin's front door both inside and outside the house. These owls are a reference to Merlin's pet owl Archimedes, a character from Disney's 1963 adaptation of "The Sword in the Stone", who does not appear in the Kingdom Hearts series.

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.