There is a rare Reaction Command that can only be seen on the first ever visit to Hollow Bastion. In the event battle where Sora and Leon must destroy a posse of determined Nobodies in the Bailey and prevent them from breaking through the gates to the castle and town, in the rare occurrence that Leon manages to get knocked out, Sora can use a Reaction Command called "Recovery Run" to revive Leon while also spinning and attacking around him.

Twilight Town had a radically different design in early concepts that featured a more fantasy-like setting with a fairy-tale-forest look that was complete with swampy ponds, grasslands, a train system that runs on large vines, and settlements atop incredibly large mushrooms.

In a Kingdom Hearts Ultimania for this game, there was an early design for Roxas that was shown off where he had hair and attire that greatly resembles Sora's. Spoiler:One possible reason for the change was to better hide the fact that Roxas is Sora's Nobody.

In areas where non-playable party members are walking around independent of Sora (i.e. the Bamboo Grove for instance), if the player looks at both Donald and Goofy from a distance they will sometimes walk toward each other and have a silent conversation. If Sora gets closer and approaches them, then they stop and focus their attention on him instead.

If the player has themed Command Menus turned on, "Hidden Mickeys" (a recurring symbol of the shape of Mickey Mouse's head inserted in plenty of Disney releases, theme parks and iconography) of varying levels of obscurity can be spotted in many of the area-specific menus. Some notable examples include:
•The gold gladiators on the Olympus Coliseum world's menu hold shields shaped liked Mickey's head
•In the middle of one of the gears in Hallow Bastion's Menu is a Mickey head
•The hard-to-see puffs of smoke "rising" out of the Underworld's Menu are translucent, wavy Mickey heads.

There is an unused version of the song Lazy Afternoons that's more orchestrated. This is likely an earlier version of the song.

Found within the game's data is what appears to be a song based on "Eye Catching" from Final Fantasy IX, which is suggested to have played when Sora meets Vivi.

In the game there is leftover code for some of Sora's animations he used in Kingdom Hearts. This includes stances, walking, attacking and jumping.

In the Japanese version, when you're done fighting the Nobodies with Axel to get to "The World That Never Was" in "Twilight Town" (3rd visit), there's a cutscene of Axel performing his special move which he looks like he's on fire and after he's done, there's a bit of fire around him. This was removed on all International versions of the game.

In the Japanese version, before the fight against Captain Barbossa and the Illuminator in "Isla de Muerta, Port Royal" (1st visit), a cutscene plays of Jack Sparrow impaled by Captain Barbossa's sword off-screen, unlike in the film "Pirates of the Carribean: The Curse of the Black Pearl" in which it is shown on-screen. In the International release however, the sword was removed entirely.

In the Japanese version there's a cutscene which shows Donald Duck getting spanked by Daisy. This scene was changed in the International version to have Donald getting scolded instead.

In the Japanese release, after fighting the undead pirates during the first visit on The Interceptor in Port Royal, there's a cutscene in which Will Turner positions a pistol up to his head, as he does in the film "Pirates of the Carribean: The Curse of the Black Pearl". This scene was changed in the international release to have Will point his gun to the ground instead.

Yen Sid spoke for the first time in an episode of the "Wonderful World of Disney" (1971) called "Disney On Parade", in which he was voiced by Michael Rye. At the time his name was unknown, and he was referred to as "Merlin" by mistake. But his name was corrected when Kingdom Hearts II came out, almost 35 years later. He is voiced by Corey Burton, who still does Yen Sid's voice in other Disney animations.

Yen Sid's name is simply "Disney" backwards, and is supposed to represent Walt Disney himself.

Roxas has unused quotes in the game that can be activated when Spoiler:Guiding Minnie to the Hall of the Corner Stone so Sora, Donald, and Goofy can fix it. This suggests that Roxas may have once served a bigger playing role, or it was simply used for testing.

Despite Kanga's voice actress at the time, Kath Soucie, being a part of the game's cast, she surprisingly did not voice Kanga in the game.
Instead she was cast as Sally and Shock of the Halloween town world.

The voice actress for Vivi Orunitia, Melissa Disney, is a distant relative to Walt Disney. They are fourth cousins.

The title card for Halloween town contains an altered version of Kefka's laugh from Final Fantasy VI. The original Kingdom Hearts also has this, although its faster and harder to notice.

In the Proof of Existence area of the Castle That Never Was, there are panels for each of the Organization members, represented by that member's weapon. However, Zexion's panel is so damaged, it's impossible to see his weapon. This is due to his weapon not being revealed in Kingdom Hearts: Chain of Memories, as the player never fought him.

Cloud's appearance in Kingdom Hearts II was based off of his appearance in Final Fantasy VII: Advent Children.

This was Pat Morita's final acting role before his death in 2005. He was reprising his role as The Emperor of China from Mulan.

Members of Organization XIII were supposed to be enemies in Olympus Coliseum battles, but were left out of the original game's release due to time constraints. They were later included in the Final Mix version of the game.

According to Tetsuya Nomura, the idea to put Sea-Salt Ice-Cream in the game came from the time he went to Tokyo DisneySea, where they sold this type of ice cream. He stated he liked the treat so much, he asked Disney if he could put it into the game as a treat the kids in Twilight Town ate during summer vacation.

Through the use of a cheating device, 2 alternate versions of the Ultima Weapon can be found. These weapons are titled "Edge of Ultima" and "Detection Saber". In game they are given a glitched appearance and are only palette swaps of the original Ultima Weapon. They are also missing a menu picture, and instead a map is displayed.

According to the trailer, the castle at Hollow Bastion from the first game may have been where Sora first met Organization XIII, instead of in the bailey like in the final version.

The Fenrir Keyblade's appearance in Kingdom Hearts II is based on a pin tumbler lock key unlike all other Keyblades. The name comes from Norse mythology and is also a reference to Cloud's motorcycle in Final Fantasy VII Advent Children.

Chicken Little was included in the game as a summon to promote the movie, which had not been released in Japan at the time. He is also the first CG movie character to appear in the series.

Xigbar's weapon was changed for the American release. In the Japanese version, he carried a weapon that resembled a sniper rifle and his scope included a cross-hair while zoomed in. In the American release, his weapon was changed to look less like an actual rifle. His weapon was split into two blades, and his scope was changed to include a three circles design instead of a cross-hair. This is one example of many changes from the Japanese version.

In a beta picture of "The Battle of the 1000" at Hollow Bastion, there appears to be two Behemoths standing in the middle of the Heartless army with Sora, Donald and Goofy charging at them. This would imply that you would fight more types of Heartless than the Armored Knights and Surveillance Robots. Also that Donald and Goofy would be a part of the battle. According an interview with the director, Tetsuya Nomura, the battle was so big that the team almost dropped it from the game even without the extra Heartless, due to the sheer quantity of enemies on screen.

Gaston's absence from the "Beauty and the Beast" world (Beast's Castle) makes him the first Disney villain to not appear in the Kingdom Hearts series with a world based on his movie featured as a playable level.

Originally, the final boss, Xemnas, was going to have another final form.

Toy Story's Woody and Buzz were set to appear as summons in Final Mix, but were removed for unknown reasons. These models were found in the code, untextured, and amongst the game's summons.

Selphie incorrectly refers to Tidus as "Tie-Dus" instead of the correct pronunciation of "Tee-dus" during a cutscene with Kairi.