In the castle courtyard in Super Mario 64, there is a plaque to a power star with an illegible name written below, often interpreted as either "Eternal Star" or "L is Real 2401" - with the latter being more popular for how it could be seen as a cryptic hint towards unlocking Luigi.
The South Korean version of the game (released two years after the original Japanese edition) contains eleven unused maps not found in any other release, featuring fully 3D environments which do not line up with any locations present in the finished product. All assets related to these maps are dated after the game's Japanese release, with intervals ranging from five days to just over three months. Additionally, the maps' texture names are written in Romanized Japanese rather than Korean, indicating that they were not created by Nintendo of Korea.
Two of these maps, kri_04 and kri_05, additionally feature various cat NPCs, all drawn in substantially different art styles compared to not only each other, but also the final game. Each one is named after a developer from the Super Paper Mario staff: yamada_neko02 (Koichiro Yamada), koba_neko (Sayuri Kobayashi), tuka_neko (Naoko Tsukamoto), and kawa_neko (Chie Kawabe).
Of these four, kawa_neko is the most unique, and was apparently designed as a player character. Firstly, the cat's name is only given to its mesh, with its sprite instead being named bc_all.1. Additionally, kawa_neko features an animated tail and a mesh that is centered on the ground rather than the middle of the room. Furthermore, new_neko_18, a redesigned version of kawa_neko with white fur instead of black, can be found in kri_08, kri_09, and kri_10; new_neko_18's mesh is explicitly labeled "PLAYER" in the data for these maps.
Taken together, all of these elements imply that these early rooms were created as a proof-of-concept for an original project by Intelligent Systems that ended up cancelled for unknown reasons.
The source code for Doubutsu no Mori contains references to a variety of non-Nintendo Famicom ROMs that would not appear in the final game, including Arkanoid, F1 Circus, and most bizarrely, the bootleg port of Tekken 2 by Hummer Team.
Felix the Cat, the new playable character announced alongside the Kickstarter campaign for the game's story mode DLC RULE of Inverse Ninjas, was decided through a Twitter poll. The other three characters on the poll were The Continental Op (a private investigator character created by Dashiell Hammett in 1923 whose copyright expired in 2019), Gnorm Gnat (an insect character created by Jim Davis prior to Garfield who entered the public domain as a result of his original comics not having a copyright notice), and Tarcis the Lover (a joke slot seemingly originating from a dating simulator made by this game's co-developer Trainwreck Studios, and that voting for was actively discouraged by the developers).
Super Mario Bros. / Duck Hunt / World Class Track Meet has unused graphics from Super Mario Bros: The Lost Levels, which can be activated by a Game Genie code, allowing one to play SMB1 levels with The Lost Levels graphics, albeit with some glitched tiles in different parts of the game.
Inside the code of Cheggers' Party Quiz and Alan Hansen's Sports Challenge, there are two unused test videos, unusually taken from the 2006 direct-to-DVD movie Bratz: Passion 4 Fashion Diamondz.
Game & Watch Gallery 3 has a crude unused placeholder title screen with Wiggler on it. This already unused piece of content itself has unused sprites of its own that don't show up when the screen is activated - including Fly Guys, flowers, a crocodile, sleeping men, and - very morbidly - Mario and Luigi roasting a whole rotisserie Pikachu.
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Dottori-Kun, a 1991 demake of Sega's 1979 maze game Head-On, was created not to be played, but rather as a legal loophole around Japan's Electrical Appliance and Material Control Law which claimed that all arcade machines must contain a game when sold. Dottori-Kun allowed Sega to sell generic Astro City arcade machines which the arcade owner could swap the contents of at their leisure, and included test features to ensure the machine worked properly before installing a proper game. The game did not support coin insertion, and therefore was not a viable option for arcades even if an owner believed there was an audience for its simplistic gameplay and graphics, leading to the board being scrapped most of the time.
Despite Floigan Bros. being initially developed prior to the Dreamcast's release in 1999 (of which the main characters Moigle and Hoigle made cameos in the Dreamcast advertising campaign "It's Thinking"), and then-President of SEGA's American division Bernie Stolar saying that "Floigan will do for SEGA what Mario did for Nintendo", the game would go through developmental setbacks until finally releasing on July 30, 2001, months after SEGA ceased production on SEGA Dreamcast.
Because of this late release window, several pieces of monthly on-disc DLC for the game, as well as the concept of exchanging Moigles through the Dreamcast VMU, were left on the cutting room floor.
In Super Fall Brawl, one of the many reskins of Jingle Brawl, the new character was intended to be Sheen Estevez from Jimmy Neutron, in promotion for his then-new spin-off series Planet Sheen. However, Sheen was not coded correctly, making him near-impossible to beat in a fight. The game's second update would remove Sheen and his stage completely, and he would never be added back in, making Fall Brawl the only Super Brawl game without any new combatants. Sheen would return in Super Brawl 2, though the House of Pain stage, based on the first episode of Planet Sheen, would be swapped for a Zeenu stage generally based on Planet Sheen instead of a single episode.
An early concept for Crash Bandicoot, back when he was originally known as Willy the Wombat, was to have the Cortex Vortex be a surplus of television sets used to brainwash the evolved animals into doing the bidding of Dr. Cortex. After being subjected to the Vortex's media barrage for 7 days and nights, Willy would've been given a tendency to speak and act in a series of bizarre nonsequiters that referenced classic literature, pop culture, or from radio, TV or movies. Some examples would've included "Loose lips, sink ships!", "But brushing with Best Toothpaste gives white, white, white teeth!", and "Don't fire boys, until you see the whites of their eyes!" His demeanor was similar to other hyperactive pop-culture referencing comedy characters from the late 80s and 90s, where he could be quoting from Rambo one moment before switching to Fred Astaire, to John Wayne, and then to Fred Flintstone.
During the game's development, Lucas - the protagonist of Mother 3 - was briefly considered as a replacement for Ness by the development team. However, due to the delays surrounding Mother 3's original Nintendo 64 release (and that game's eventual cancellation), Ness returned instead - as was originally planned.
Both Ness and Lucas would go on to be playable in this game's sequel, Super Smash Bros. Brawl, though Lucas's appearance would be based on Mother 3's eventual Game Boy Advance release as opposed to what is now commonly referred to in the fan community as "EarthBound 64".
In the game's code is evidence of an unused Event Pokémon. Event Pokémon are exclusive Pokémon throughout the franchise's history which usually have movesets or other unique properties that would not otherwise legitimately occur in normal gameplay.
A code string indicates that Diglett learns the move "Acid" at Lvl. 1, found at the end of Diglett's Generation 1 move pool, as this programming method was the manner in which the programmers ensured Event Pokémon transferred via the Game Pak to Stadium could function properly during gameplay.
To date, a Diglett with Acid being legitimately learned in it's moveset has never been released or distributed by an official Event by Nintendo nor Game Freak.
18 years after the game's release, on September 16th 2017, a Silent Hill fan community member by the name of roocker666 posted images of never before seen enemies hidden in the game's files online. These formerly unseen creatures include a giant headless frog (FRG), a possibly monkey based creature (MKY), an ostrich like creature (OST), a purple moth (BTFY), a manta ray (El), and a faceless snake (SNK).
Three additional models were discovered at a later date, including beta versions of the Larval Stalker and the Grey Children, and most notably a giant chicken (CKN) that was bigger than the others and is thought to have been intended to be a boss character.
Masahiro Ito, the game's background and creature designer, acknowledged the discovery on Twitter, but had no comment regarding the original purpose of the models.
In an interview with Satoru Iwata and Shigeru Miyamoto, it was revealed that an early, experimental build of the game had split-screen multiplayer, with Mario and Luigi as playable characters.
The game's planned multiplayer mode ended up being reworked into a Mario Bros.-style minigame thanks to the Nintendo 64's technical limitations making the split-screen concept unfeasible. However, in February of 1996 (just a month before E3 of that year), both multiplayer mode and Luigi were removed, in part because the Nintendo 64 was bundled with only one controller. These statements are corroborated by findings from the Gigaleak, a massive content leak of internal data from Nintendo in 2020 that included early assets from Super Mario 64. Among the leaked material was a model and textures for Luigi and sprites for "1P" and "2P" icons. Luigi's model is dated June 20, 1995, while the icons use the same generic font seen in the Shoshinkai demo from November of that year, rather than the custom font seen in both the E3 1996 demo and the final release.
The first 100,000 copies of the game to be printed for the Playstation contained a hidden video file that couldn't be accessed in-game. The video was the South Park pilot "Jesus vs. Santa".
Wuhu Island was originally created for Wii Sports before its debut appearance in Wii Fit. An early version of the island can be seen in a leaked development image for Wii Sports, and in footage of the unused Airplane sport in the E3 2006 demo for the game. The leaked image also indicates that the Airplane sport would have also given you the option to fly around the island with a Rocket Belt, an idea that would later be reused in Pilotwings Resort.
In the GameCube and Xbox versions of Harry Potter and the Chamber of Secrets, it was intended for there to be portraits littered throughout Hogwarts of various real world people, specifically developers, their families, and some people the developers forgot about years after the fact (but believed they were involved in some kind of contest). The portraits of family members in particular were included as a coping mechanism, as they were suffering during development.
"We were doing very long hours, and it wasn’t just us suffering – our families were, too. They were supporting us – often suffering in silence. Marriages were strained, children were missing a parent. It seemed only right to put their pictures into Hogwarts; our odd way of showing our love and gratitude. And possibly to remind us what they looked like."
The development team considered these portraits some of the very few elements of the game they were truly proud of. Eurocom requested the portraits be removed without explaining why, something the developers were "too tired to fight".
According to leaked development files from The Simpsons: Hit & Run, Sideshow Bob and Poochie were originally planned to appear in the game. There is an unused low-polygon model for Poochie that is pre-posed, meaning that it was likely supposed to appear as merchandise of the character (most likely alongside Itchy and Scratchy's balloons in Level 2), rather than appearing in an Itchy & Scratchy cartoon cutscene. No models for Sideshow Bob are known to exist.
While Super Donkey is thought to have ultimately evolved into Super Mario World 2: Yoshi's Island, the project was likely tossed around as a Mario or Zelda game at various points in development.
Various development assets associated with Super Donkey, which were uncovered alongside the prototype in the 2020 Gigaleak, include sprites and animations for Link, Mario, and Donkey Kong; the animation files for Link are dated to fall 1990, while Mario's animation files are dated to winter of that year. Additional files for all three characters span overlapping periods in 1991. Link's sprites and animations indicate that Super Donkey was at one point planned to be a side-scrolling Zelda title similar to Zelda II: The Adventure of Link, while Mario's animations imply that the iteration that starred him and Donkey Kong eventually morphed into the 1994 Game Boy title Donkey Kong.
In the castle courtyard in Super Mario 64, there is a plaque to a power star with an illegible name written below, often interpreted as either "Eternal Star" or "L is Real 2401" - with the latter being more popular for how it could be seen as a cryptic hint towards unlocking Luigi. 
The South Korean version of the game (released two years after the original Japanese edition) contains eleven unused maps not found in any other release, featuring fully 3D environments which do not line up with any locations present in the finished product. All assets related to these maps are dated after the game's Japanese release, with intervals ranging from five days to just over three months. Additionally, the maps' texture names are written in Romanized Japanese rather than Korean, indicating that they were not created by Nintendo of Korea.
Game & Watch Gallery 3 has a crude unused placeholder title screen with Wiggler on it. This already unused piece of content itself has unused sprites of its own that don't show up when the screen is activated - including Fly Guys, flowers, a crocodile, sleeping men, and - very morbidly - Mario and Luigi roasting a whole rotisserie Pikachu.
An early concept for Crash Bandicoot, back when he was originally known as Willy the Wombat, was to have the Cortex Vortex be a surplus of television sets used to brainwash the evolved animals into doing the bidding of Dr. Cortex. After being subjected to the Vortex's media barrage for 7 days and nights, Willy would've been given a tendency to speak and act in a series of bizarre nonsequiters that referenced classic literature, pop culture, or from radio, TV or movies. Some examples would've included "Loose lips, sink ships!", "But brushing with Best Toothpaste gives white, white, white teeth!", and "Don't fire boys, until you see the whites of their eyes!" His demeanor was similar to other hyperactive pop-culture referencing comedy characters from the late 80s and 90s, where he could be quoting from Rambo one moment before switching to Fred Astaire, to John Wayne, and then to Fred Flintstone.
In the game's code is evidence of an unused Event Pokémon. Event Pokémon are exclusive Pokémon throughout the franchise's history which usually have movesets or other unique properties that would not otherwise legitimately occur in normal gameplay.
18 years after the game's release, on September 16th 2017, a Silent Hill fan community member by the name of roocker666 posted images of never before seen enemies hidden in the game's files online. These formerly unseen creatures include a giant headless frog (FRG), a possibly monkey based creature (MKY), an ostrich like creature (OST), a purple moth (BTFY), a manta ray (El), and a faceless snake (SNK).
In an interview with Satoru Iwata and Shigeru Miyamoto, it was revealed that an early, experimental build of the game had split-screen multiplayer, with Mario and Luigi as playable characters.
Wuhu Island was originally created for Wii Sports before its debut appearance in Wii Fit. An early version of the island can be seen in a leaked development image for Wii Sports, and in footage of the unused Airplane sport in the E3 2006 demo for the game. The leaked image also indicates that the Airplane sport would have also given you the option to fly around the island with a Rocket Belt, an idea that would later be reused in Pilotwings Resort.
In the GameCube and Xbox versions of Harry Potter and the Chamber of Secrets, it was intended for there to be portraits littered throughout Hogwarts of various real world people, specifically developers, their families, and some people the developers forgot about years after the fact (but believed they were involved in some kind of contest). The portraits of family members in particular were included as a coping mechanism, as they were suffering during development.
According to leaked development files from The Simpsons: Hit & Run, Sideshow Bob and Poochie were originally planned to appear in the game. There is an unused low-polygon model for Poochie that is pre-posed, meaning that it was likely supposed to appear as merchandise of the character (most likely alongside Itchy and Scratchy's balloons in Level 2), rather than appearing in an Itchy & Scratchy cartoon cutscene. No models for Sideshow Bob are known to exist.
