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.

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.

The ending to the beta trailer for the game shown at E3 2001 featured an alternate logo. The word "Kingdom" is shown in normal golden text while the word "Hearts" is displayed in a modified version of the final logo's spiky text. The trademark heart is also missing from this version of the logo.

Bahamut, a recurring summoned creature from the Final Fantasy series, was planned to be a summon for Sora. But it was removed from the final version. Searching through the game data, a Bahamut summon option can be chosen through the use of cheat codes. However, since there is nothing to execute when it is chosen, the game will crash.

The secret ending, "Another side, Another story," was not meant to be a real game. It was only there to show off ideas that Tetsuya Nomura had for the sequels. It was not until Kingdom Hearts 358/2 Days that the fight shown was playable.

Disney Castle was supposed to be playable. It was shown in a beta trailer but the level was cut from the final version. However, through cheat codes, it can be accessed.

According to some concept art released by Square Enix, Sora was originally not going to wield a Keyblade at all, but a chainsaw sword instead. He was also going to have a design with lions fur, claws, a tail and ears.

In the Japanese version of Kingdom Hearts, the Chernabog fight does not have the "Night on Bald Mountain" music. Instead the regular boss music will play. The "Night on Bald Mountain" music was added to the international release to make the scene more true to the scene from Fantasia. The music was added into the Final Mix version of the game.

If you enter the Hercules Cup time trial, get to Hercules, pause the game and choose to restart the time trial, it will take you back to the start of the Hercules fight and the timer will be frozen at 00'06"23.

There was a planned boss battle with Riku who would be possessed by Ansem in an area titled Crumbling Island at the End of The World which was based off of Destiny Island, but devoured by the darkness.

In Deep Jungle, outside of the tents on a table is a pot and cup which bare a resemblance to Ms. Potts and Chip from Beauty and the Beast. These characters also appeared in the Tarzan film.

Completing the world "Deep Jungle" before Wonderland will cause Snow White to be kidnapped instead of Alice in the cut scene which is shown after completing Deep Jungle.

The enemy "Assault Rider" was meant to appear in the game but its style didn't fit any of the worlds so it was removed and used in Kingdom Hearts 2 instead.

Whenever Sora wins a round in the Olympus Colosseum, he performs a small victory pose. These animations are based off of Cloud's victory poses in Final Fantasy 7, and Squall's from Final Fantasy 8.

Sora was not originally the main protagonist of the series. In the beginning, Disney had desired Donald Duck to be the protagonist, while Squaresoft wanted Mickey Mouse. Series creator Tetsuya Nomura wanted neither and developed an original character based off of Disney's style. In the end, Sora was created. His yellow shoes, red shorts and white gloves are based on Mickey Mouse.

An early trailer reveals that Donald and Goofy's costumes were originally different.

Before the game was released in the U.S., Square held a "Name-in-Game" contest. A person named Kurt Zisa won the contest, and his name was used for a hidden boss in the game.

The idea for the game came from an elevator ride with Shinji Hashimoto (Final Fantasy and Square producer) and a Disney producer, who shared the same building in Tokyo. They came up with a crossover as a vague production idea, and the head of Square-Enix, Hironobu Sakaguchi, later agreed to the idea.

In an early trailer for the game, there is a scene showing Riku on top of Big Ben in Neverland. This scene does not appear in the final game, nor does Riku ever go to Big Ben in any way. Despite this, it is considered canon as a cutscene in Kingdom Hearts Re:coded shows that this is where Riku and Kairi went after leaving Captain Hook's ship.

The Pride Lands were originally going to be to be a playable level, with Sora turning into a lion to fight. This idea was scrapped as fighting and walking in a four-legged body required additional programming that the team did not have time to create. The concept was later used in the sequel.