Kingdom Hearts

In-game reset:
Hold R1 + L1 + Start + Select + R2 + L2 during game play.
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Alternate ending sequence:
Successfully complete the game with all Keyholes locked (including The Book Of Pooh), 99 Dalmatians rescued, and the Hades Cup won.
GodlyWolf and elise.godin.

Successfully complete the game under the expert difficulty setting and view the secret trailer. A different screen will appear at the end.
Brianna Martinez.

Hidden trailer:
Wait until the credits complete to view an epilogue of what happens to Sora, Donald, and Goofy after the worlds come back. It seems to be a teaser for another Kingdom Hearts game. After that, there is a "Special Secret" which seems to be a hidden trailer for yet another Kingdom Hearts game, although this one seems to be about Riku and what happens after the defeat of Ansem and the Heartless.
Super Guy.

Note: The hidden trailer after the one with Pluto and the letter is actually a portion of the trailer from Kingdom Hearts 2. The Japanese game Kingdom Hearts Final Mix should show half of that trailer.
UYH770.

To view a hidden trailer, remain idle at the start screen for slightly less than a minute. An intermission sequence will begin. Wait with the controller remaining idle to see the trailer.
cloakwarrior65.

Select Opponent option:
When you defeat Sephiroth, a new option (Select Opponent) in Hades Cup will open. This lets you select an opponent to fight with a lot of experience points.
Pikachu Booty.

Expert mode:
At the beginning of the game when asked to choose the difficulty setting, select "Difficult" .When you are in the room with the sword, staff, and shield, choose the sword. This is considered expert mode. At the end of the game, you will get to see your overall statistics and some artwork
NeoEP.

Glitch : 100 Acre Woods: Make Sora fly:
Stand at the bottom edge of the Winnie The Pooh book and glide/superglide towards the bottom. Sora will fly upwards and disappear. He will reappear after a few seconds of flying.
MonkeyDude2.

Glitch: Agrabah: Make Sora fly:
Go to the cave after the desert. Go to the treasure room. Immediately when you enter, there should be a pile of gold. If you have High Jump equipped, unequip it. Go to the pile of gold directly in front of you, Jump to the side closest to the door where you entered and press Analog-stick Up. Sora should now be flying. Note: As soon as you release the Analog-stick, Sora will stop flying.
mario gonzalez.

Glitch: End Of The World: Floating Sora:
Go to Destiny Islands in the End Of The World. Go above the sea shack. Make sure you have Super Jump equipped. Go to the edge close to the tree and jump. Eventually you will jump on an invisible path. Keep pressing Circle to go higher. Note: This will require several attempts. This glitch also allows you to walk through walls.
Jacob Allen.

Glitch: Neverland: Go nowhere:
Go to the bottom of the ship where the save point is located and get on a ladder almost to the top. Change the view to where you can see Sora's legs and arms from a side view. Press the Left Analog-stick far to whichever direction Sora is located. He will be doing a climbing motion but not moving up or down. Note: Try again if it does not work the first time.
Lexi Blake.

Glitch: Invisible Gummis:
Hold L1 + L2 + R1 + R2 for about ten seconds when you warp to a world. You will only see your weapons, your cockpit, and your engine moving. Note: Save the game before attempting this trick, as the game will reset afterwards.
Coasterkid19.

Glitch: Change Keyblade appearance:
Note: This only works with the Kingdom Key (your initial equipment) and the bottom keychain on your list. Go into the equipment menu and equip the Kingdom Key. Exit the menu and withdraw your Keyblade. Go back into the menu and toggle between the Kingdom Key and the last keychain on your list. You can do this easily by pressing Down from the bottom item and Up from the Kingdom Key. Do this slowly at first, then with enough speed that the Keyblade's picture does not change from the Kingdom Key. Stop on the bottom key (usually your most powerful) and very quickly equip it. Exit the menu. Your Keyblade's appearance should not have changed, but, in effect, you have the more powerful Keyblade equipped. You can fight with the appearance of the Kingdom Key for one area screen, then it will revert to its normal appearance. You can do this in reverse also: equip the Kingdom Key but have the appearance of the more powerful key. This is not really something that affects how you play the game, but it can be used in the final battles against Ansem for the effect of having the "correct" keychain.
Megumi.

Glitch: Roll through Tiger Head:
Before the battle with Tiger Head, equip Dodge Roll on Sora then head to the desert. When Tiger Head appears from the sand, jump onto the back of his neck and run towards him. Use Dodge Roll and you should roll through him. This is quite useful, as you can now jump on top of his head without waiting for him to bury his head into the ground.
Musashi1848.

Glitch: Get to chest behind magician's house without glide:
After getting Mermaid Kick from Atlantica. go behind the magician's house in the water. Go directly behind the rock and do Mermaid Kick. Sora will jump onto the rock.

Glitch: Hercules description:
In the character section, look at Hercules. It states that he is the son of Zeus and Hera. In fact, he is the son of Zeus and a mortal. Hera, who is the queen of the gods, resents him for Zeus' cheating.
Azmodan0588.

Glitch: Sephiroth and Cloud dancing:
When in a fight against Sephiroth or Cloud in the Olympus Coliseum, wait for them to jump or they are about to jump. Pause game play. If done correctly, when you choose "Restart" on the pause menu Sephiroth and Cloud will start jumping continuously without touching the ground and appear as if they are dancing. Note: This may require some practice. This works better when Cloud is alone because Leon may be a distraction. It does not work with Cloud if he is doing the jump that leads to impaling the ground with his sword.
FLBDVM.