Acting for Two: Xion shares her Japanese voice actress with Kairi, and in Days Stoner also plays The Other Darrin for Kairi. However, Kairi's original voice actress, Hayden Panettiere, returned for Birth by Sleep... as Kairi and as Xion, interestingly enough.
Actor Allusion: Eraqus's Knight Templar behavior and attitude towards darkness and light are very similar to how the Jedi in the Star Wars prequels are portrayed, particularly in how Terra desires greater power to protect his friends, and Eraqus warns that type of obsession and fear leads to darkness.
Beam Me Up, Scotty!: A lot of fanart, fan-animation, etc. depict the keyblade being swung with the teeth facing forward, like an ax, when in the actual games it's swung with the teeth facing backward, like a blunt-force object. A side-effect of this is that Ventus' default keyblade, Wayward Wind, is often depicted in fanart with the teeth facing the wrong way, which is very noticeable given Wayward Wind's asymmetric design.
Cash Cow Franchise: How else do you explain the presence of five side story games between KH2 and 3, one of which existing just to tie up all the loose ends left by almost every other game? Given the series's enthusiastic fandom and the fact that it's made by both Square Enix and Disney, it's not surprising.
Several characters have lost their voice actors over the years, but the most prominent ones are Mickey Mouse and Master Xehanort (the former is Wayne Allwine; one of the games in the series is his very final Mickey Mouse voice role, having been released posthumously). The latter is very noticeable, as both his English and Japanese voice actors, Chikao Otsuka and Leonard Nimoy, died a month apart in 2015! Then in June 2015, Sir Christopher Lee passed away, but Corey Burton had been filling in for Lee since 2010.
Averted in that while Robin Williams passed away in 2014, Dan Castellaneta, who replaced him for Aladdin: The Return of Jafar, has always been playing The Genie in these games note they started after Bicentennial Man created a rift between Disney and Williams; he only voiced The Genie twice, first in the original Aladdin film, then in Aladdin and the King of Thieves. Jim Meskimen also played the character in the 2.5 cinematic version of Re:coded.
Disney Adventures, the official magazine for The Walt Disney Company, inexplicably delivered a totally inaccurate blurb on the first game's plot, claiming it was about Disney villains trying to change the endings of their stories so the bad guys win. This was the premise of the slightly older Disney's Villains' Revenge, but doesn't really apply to Kingdom Hearts.
In Italy, Disney's official materials (articles on the Italian Mickey Mouse and their strategy guide) about the first game of the series claim that "the invincible Heartless prince" Darkside (the giant Heartless Sora fights at the beginning of the game and later before the final boss) is the Big Bad of the game, and that the game ends with Sora fighting him in Neverland. In the actual game, Darksides are a generic heartless (note the plural), and while Sora does fight one again later on, it's in the remains of the Destiny Islands, not Neverland. Since you encounter it in the middle of fighting the game's realBig Bad, Ansem, it may have been a simple misunderstanding.
Cursed Item: Out of universe example; but in this case, it's a song. According to Yoko Shimomura, the chorus portion of the iconic "Destati" is believed to be cursed because every time the developers loaded it into the game, something bad would happen. An example of this was the entire building suffering a power outage.
Haley Joel Osment, Hayden Panettiere and David Gallagher have become this in the course of production. When the first Kingdom Hearts came out, they were 14, 13 and 17 respectively, playing characters who were 14, 14, and 15 respectively. However, by the time Dream Drop Distance came out, ten years had passed in real life while only two have passed in-universe.
Chain of Memories, originally a mostly unvoiced GBA title, was reworked into a full PS2 game as Re:Chain of Memories in 2007 - after the release of Kingdom Hearts II. Thus, 19-year-old Haley Joel Osment appears playing the same version of the character he voiced at 14 to a rather jarring result. Averted with the only other teenaged character; 14-year-old Naminé is played by 15-year-old Meaghan Jette Martin.
Brittany Snow was 20 when she voiced the 15(ish) year old Naminé in Kingdom Hearts II.
Surprisingly, Square Enix was able to produce a HD remaster for the original game and retain Deep Jungle, which would be otherwise impossible without cutting the world out. And since some items relating to the controversial world have appeared in coded, it's possible that it's a sign saying that the trope could be averted one day.
Enforced in the games themselves to a certain degree; Disney has strict rules about how the franchises interact. This leads to both The Stations of the Canon (in that the plots of the levels loosely follow the movies they're based on) and an Alien Non-Interference Clause (in that the natives of each world can't be told about other worlds). The rule of thumb seems to be that if you're tied to a particular movie, you're Locked Out of the Loop, though some characters get a bit of leeway in this, including The Beast being a partner during Hollow Bastion in Kingdom Hearts I, Cloud, Zack and Auron being plot-critical in the Colosseum stages in 1, Birth By Sleep, and 2, and the TRON level being Hollow Bastion's operating system in Kingdom Hearts II.
In a sense, Kingdom Hearts V-Cast was made by Disney Mobile, so while it makes sense that he'd appear in Wonderland's chapter from the Disney side of things, the Alice in Wonderland Caterpillar isn't considered canon because Square-Enix's games don't feature him in Wonderland. Despite this, he appears in the manga adaptation for Days.
Mysterious Figure, which is from the Brady Games Birth by Sleep guide. The name is unofficial, but persists in the fan community for the sake of distinction from the original Unknown.
A handful of fans also refer to him as Soranort. Because at this point, he's bound to be related to one of them somehow. Ironically his entire plan in 3D is to create/become a literal "Soranort."
As of the release of 3D, he's been referred to as Trollanort, thanks to his affinity for playing literal mindgames and his bordering-on-unfair use of time control in his boss battle. Other variants include Clockanort and "TOKI WO TOMARE-nort".
Apparently Luxord's is George Michael for the (unintentional?) resemblance.
Kingdom Hearts 2.8 has recently acquired the nickname "Kingdom Hearts 3 Ground Zeroes" due to the recent compilation serving as a prologue to Kingdom Hearts III, much like MGSV Ground Zeroes serves as a prologue to the main game.
The Riku Replica has often been called "Repliku" to give him some semblance of an original name. This is actually the name he is addressed by in the German release of Chain of Memories.
Anyone who gets Grand Theft Me'd by Xehanort gets "Nort" in their name. For example, "Terranort." The process itself has the name "Got Norted."
"Darqua" for a darkness-corrupted Aqua.
The original form of a Nobody or Heartless is called a "Somebody" by the fandom, especially among Organization XIII (i.e. Sora is Roxas' Somebody). The games don't have a specific terminology for what they otherwise just call "people" or "humans", though the fan term may come from a quote from Axel in Chain of Memories ("We nobodies can never hope to be somebodies.")
Flip-Flop of God: Though some of this is also the fault of Word of Dante, there have been conflicting explanations for why Sora and Roxas are able to dual-wield Keyblades. The current official explanation is that Sora has the ability due to harboring Ventus's heart, while Roxas inherits the ability as Sora's Nobody. This is a direct flip of what fans had previously assumedthat Sora got the power from absorbing Roxas, and Roxas in turn got the power from absorbing Xion.
The HD ReMIX Collections being ported to the Xbox One, despite the fact that III is coming to the system. Due to the collections being put together for the specific purpose of catching people up to speed on the games prior to III, it's incredibly odd that Square hasn't brought them to the system yet. Nomura has said that he's not against the idea, but they had to gauge interest and wait till after III comes out to be able to port them due to wanting to not have III delayed. Then you have Tai Yasue saying that they still have yet to make a decision on that and Nomura saying that they still need to talk to Microsoft about it. While ports seem to be very slowly coming along for them, it's left a lot of fans confused on them.
God Never Said That: A main reason that most PS4 fanboys claim that the games prior to III haven't been announced to be coming to the Xbox One yet is because of a supposed exclusivity deal with Sony for the first two main games to stay on PlayStation. Not only does this not make any sense, seeing as the series has been multiplatform since Chain of Memories, which was originally on the GBA, but no one at Square has said anything of the sort. The most probable reasons for those games being on PlayStation, and nowhere else, thus far is that the PS 2 was the most popular gaming console of the 6th generation and Square has a good working relationship with Sony. The fact that Nomura has said that it's entirely possible to port the collections to the Xbox One at some point in the future pretty much confirms this.
No Dub for You: The first two PS2 entries were dubbed into multiple European languages, but all of the following games used the English voice track in European regions - including the HD Remix compilations, despite said European dubs existing for the older games included.note To be fair, including said dubs would have been inconsistent with the newer games in the compilations, and the HD version of KHII Final Mix+ includes newly recorded English voice acting for the added cutscenes.
III isn't getting any dubs outside of the English dub subtitled in other languages either.
Kingdom Hearts was for a time infamous for including plot-relevant content in Japan-only Final Mix re-releases. Eventually all this content reached the international audience in the form of the 1.5 HD Remix and 2.5 HD Remix compilation disks.
coded was almost an example, being a Japanese mobile phone game. This was slightly less painful than the Final Mixes since it's less important to the overarching plot, but it still has a few important scenes and offers interesting reflections on the series' themes. It was eventually remade as Re:coded for the Nintendo DS and given an international release.
Quite a few. A bad example being Mandy Moore replaced by the absolutely wooden Mena Suvari. A good example would be Lance Bass being replaced by George Newbern (who made it clear that the line is "Descend, Heartless Angel", not "Sin Harvest" as Bass' rushed delivery had many believing, including Dan Birlew, author of the official strategy guide.)
Corey Burton is The Other Darrin to a few of characters, most notably DiZ. He played MCP and Sark in II when those characters original actor, David Warner, was unable to work on the game due to a previous committment.
Before Christopher Lee's passing in June 2015, Corey Burton filled in for DiZ whenever Lee wasn't available. But from 2010 onwards, Burton has taken the role full time, especially after Lee's passing.
Xion is an unusual example. In the English version of 358/2 Days she is initially voiced by Alyson Stoner, who was The Other Darrin to Hayden Panettiere's Kairi. Panettiere would later return to voice both Kairi and Xion in Birth by Sleep and Dream Drop Distance, essentially making her The Other Darrin to Stoner's Xion. Stoner then returned in the Compilation Re-release1.5 HD Remix. See Acting for Two above.
Wayne Allwine, the longstanding voice of Mickey since the late '70s, died in May of 2009, shortly after he finished recording for Days. Birth by Sleep marks the debut of Bret Iwan as Mickey. It's a bit fitting, since Mickey is nearly a decade younger and his voice for the character sounds younger then Allwine's.
Eddie Carroll (voice of Jiminy since the mid-70's) died in 2010. Phil Snyder replaced him in subsequent games. Then Snyder quit acting and Jiminy was played by Joe Ochman in 2.5's Re:coded story.
A similar situation happened for the Genie. Initially played by Robin Williams in 2 of the films, Dan Castellaneta, who'd filled in for him in Return of Jafar and the TV show, played him in I and II. In 2.5, however, he was replaced by Jim Meskimen for the Re:coded cinematics, who was a closer voice-match to Williams than Castellaneta's already-pretty-close take. This might have been part of an overall thing for Disney properties, as Meskimen played the Genie in other Disney-themed games contemporary to 2.5's release.
Tate Donovan reprises his role as Hercules in the second game. Before that, he was voiced by Sean Astin (y'know, Sam)? In Birth by Sleep, Josh Keaton returns to voice the younger Hercules.
Brittany Snow was replaced as Naminè by Meaghan Jette Martin in Re:Chain of Memories, Birth by Sleep, 358/2 Days and Re:coded.
Additionally, since Nachi Nozawa passed away long before the third game, Vexens Japanese voice is instead provided by Shigeru Chiba.
Jack Sparrow, played by Johnny Depp in the movies, is voiced by James Arnold Taylor (the voice of Tidus and Ratchet) in Kingdom Hearts II, who does a not-so-convincing impression of Depp because of mold in his throat at the time of recording. When Jack Sparrow returns in Kingdom Hearts III, the role has now been taken over by Jared Butler. This was most likely a similar situation with the Genie, as Butler has already played Jack in Disney Infinity.
Also present to various degrees in the European dubs of the PS2 games. Although Disney managed to bring the original voice actors for their characters wherever they could, there were many of them that just weren't possible, either due to the pass of time, schedule conflicts or budget reasons.
Japanese: Chikao Ohtsuka (Master Xehanort and Captain Hook) is the late father of Akio Ohtsuka (Ansem). Incidentally, Akio Ohtsuka took over the role of Master Xehanort in Kingdom Hearts III after his father passed away.
English: Shanelle Gray (Larxene) is the older sister of Ariel Winter (young Kairi).
Shrug of God: Whether the remix collections will come to Xbox One and/or Switch (as well as III for that one) after III has come out. On one hand, there's a theory that Square is waiting to see the amount that III sells on Xbox One to justify the ports, which doesn't make sense because Xbox players going into III blind won't get what's going on in the story outside of what's happening in the main plot without them. On the other, Nomura has stated that it's a likely possibility and the main conceit of releasing the collections in the first place was to catch people up on the stories of the games in time for III to avoid massive Continuity Lockout with people who would either be playing the series for the first time with III or hadn't played the handheld games to keep up with the overall narrative of later entries' stories and setups for III, which is only more incentive for them to do it. Considering that the series used Disney characters to sell a lot of copies of a new game in the first place, the series has the potential to break a lot of ground on Xbox like it did the PlayStation compared to other Square Enix games due to the good word of mouth and hype generated by PlayStation players, so they shouldn't have much to worry about with sales numbers.
It must be said, however, that RP Gs don't sell particularly well on Microsoft systems, and the system is all but dead in Japan. Waiting to see initial sales might not be such a bad idea afterall.
Sleeper Hit: It can not be understated that the announcement of a Disney/Final Fantasy collaboration was derided by majority of gamers when first announced. Kingdom Hearts was released during a time when Disney Studios was going through a Dork Age, so most gamers felt that the collaboration would bomb. The end result was popular reviews, the game ending up as one of the top 10 selling PS2 titles, and the start of one of Square's biggest franchises.
Urban Legend of Zelda: One of the more common things you'll read online as to why the 1.5 and 2.5 HD collections haven't been ported off the PS4 yet is that the earlier PS2 games have an exclusivity deal with Sony to release on only their consoles. Not only does this make no sense, since the rest of the games, minus Birth By Sleep came out on Nintendo's handhelds, as well as mobile devices, but Kingdom Hearts III is also releasing for the Xbox One, with the possibility of a Switch port in the future. Also, Nomura and Tai Yasue have also stated that ports of the collections to Xbox One are also possibilities after III is released, but have yet to do so because they're focusing on III's development to not have it be delayed. This seems to be hearsay from some fans to justify the lack of ports of the earlier games to Xbox, despite there being no evidence to support it. In fact, there's more evidence against it.
Vocal Evolution: Due to their voice actors going through puberty, Sora, Riku, and Kairi sound very different in I than they do in the later games. Because of this, the new cutscenes added to the Final Mix version of the game are not dubbed.
The game was originally supposed to star Mickey, but there was an internal disagreement on whether Mickey or Donald should be the protagonist. The devs eventually decided to Take a Third Option and feature an original character as the protagonist.
Chernabog was to have been the original Big Bad, the embodiment of all darkness, taking inspiration from American Gods with the final battle being a gradual climb up the slopes of Bald Mountain to fight him. He was instead turned into a surprise boss due to console limitations and Executive Meddling.
The final battle with Xemnas in II was supposed be quite different, with Xemnas melding into a giant Transformer-esque creature◊ after absorbing power from Kingdom Hearts and, upon defeat, transform into this◊ distorted version. This idea was nixed and replaced with the dragon-fortress/armored Xemnas and black-and-white coat Xemnas battles.
Trailers showed variants of Behemoths and Wyverns from the first game in the 1000 Heartless Battle in Kingdom Hearts II.
The Pridelands were originally going to be featured in I, but the game engine couldn't handle any dedicated quadruped movement, so Simba was included as a summon instead. After changing the game engine, the world managed to make it into Kingdom Hearts II.
358/2 Days was planned to feature a Pinocchio world, and the game file has some Dummied Out assets. It would have included Gepetto, Foulfellow and Gideon, and human Pinocchio, and would have had a What Measure Is a Non-Human?-centered plot and a focus on circus scenes. It was cut due to space limitations. The world was eventually reworked and included in Dream Drop Distance.
The outside walls of Hollow Bastion/Radiant Garden were originally going to be playable areas in Kingdom Hearts II.
An odd example is a scene from the Beta trailer of the first game, which has Riku at the top of Big Ben. What's odd is that, despite never appearing in the finished game, it is still referenced in later games as canon.
Both Hades and Megara were set to appear in 358/2 Days.
Pleakley was meant to appear in the Lilo & Stitch world in Birth by Sleep.
Rikku was originally supposed to have been part of Leon's crew in Traverse Town, but the production team decided that her name was too similar to Riku's. She was switched out for Yuffie, who received the costume that was planned for Rikku.◊
A younger version of Irvine Kinneas was also supposed to be in Destiny Islands, but was replaced with Wakka (also more recognizable, although perhaps because of Kingdom Hearts).
Both Leon's and Cloud's pasts were going to be touched upon in Birth by Sleep, but as the world already had several members of Organization XIII appearing, it was decided that the world would have been too overloaded with characters.
Marluxia was originally envisioned as a woman, but was changed to a man when the developers realized that having two women rebelling against an otherwise all-male Organization and get killed for it was rife with Unfortunate Implications.
Axel was supposed to be killed by Roxas at the end of the prologue in Kingdom Hearts II, but was such a fan-favorite and creator-favorite character that he was brought back for the rest of the game and given a Heroic Sacrifice.
The members of Organization XIII who died in Chain of Memories (Vexen, Lexaeus, Zexion, Larxene, and Marluxia) were going to be bosses in the Olympus Coliseum in KHII, but were cut. The Data-Battles of Final Mix are a remnant of this.
Following the death of Wayne Allwine, Nolan North was considered to succeed him as the voice of Mickey Mouse. However, his busy schedule, particularly with Uncharted 2: Among Thieves, prevented him from getting the role, and Bret Iwan was cast instead.
Around 2002, Disney commissioned a Kingdom Hearts animated series... but it was passed on, despite excellent testing, to avoid contradicting future games and to respect Tetsuya Nomura's vision. Some storyboards can be seen here, and an interview with the show's creator here. Later in 2017, Seth Kearsley is going to possibly release a pilot episode on the internet very soon.
An android/iOS game called Kingdom Hearts: Fragmented Keyswas in early development, but was ultimately scrapped. Its new worlds included locations from Tangled, Wreck-It Ralph, Frozen, and even Star Wars, which Disney had only recently acquired at the time of the game's development. However, it is likely that some of the concepts featured in the unfinished work will appear in Kingdom Hearts III.
0.2: Birth by Sleep - A fragmentary passage was originally planned as the first level of Kingdom Hearts III. But due to development increasing for that game, it was moved to the "2.8 HD Final Chapter Prologue" instead.
Sora and Riku were going to wear their Kingdom Hearts I outfits in Dream Drop Distance, but instead only do in the introductory battle and first Dive sequence.
Another data mine revealed that worlds based on Toy Story, Monsters, Inc., and Fantasia were planned to be in Kingdom Hearts II. Obviously, all of those worlds ended up being in the series eventually. Also, Atlantica was going to be a full world.