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.
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.
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.
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.
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.
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 Garfield's alternate costume texture, there is a portion devoted to Garfello, Garfield's Ninja Turtle alter ego from the Garfield Meets the Teenage Mutant Ninja Turtles short comic published in the 1992 Teenage Mutant Ninja Turtles Magazine.
There is an unused model of the Statue of David by Michelangelo in Squidward's prop set. This was likely intended for his air down strong, where he slams down with a block of marble, referencing the SpongeBob episode "Artist Unknown", where SpongeBob makes the same statue by accident. The model has a clam on its groin as censorship, just like in the original episode, but does not have the nose Squidward places on it.
Slap City was originally envisioned as a mini-game in Ittle Dew 2 before being reworked into a full title. The scrapped mini-game had intentionally poor presentation, being locked to 30FPS with interlacing, something that was not retained in Slap City given it's focus on competitive play, and a low-poly style inspired by the original Super Smash Bros. The polygonal alternate form of Fluffy Fields, where the Clone Team is fought in arcade mode, is based on this version of the game.
During development of Pizza Tower, multiple demo builds were released - among these was a build known as the "Peter Griffin Experience", built off of the 2018 "early test build". This demo replaced every single sprite of Peppino with a highly compressed edit of a stock image of Peter Griffin from Family Guy made to resemble Peppino, and replaced one of the game's musical tracks with a fan-made Family Guy remix.
After this build, the Peter Griffin "arms resting" pose would appear in some builds as a taunt, albeit as an actual sprite and not an edit, and a video would be posted by developer McPig showcasing the taunt, accompanied by the first note of the Family Guy theme song, under the name "family", likely referencing a a meme video that plays the first note and ends. This taunt was removed for unknown reasons in the final game.
The Case of the Missing Kelp Seeds was the first Humongous Entertainment game to use cel animation instead of computer-designed sprites. This was a last second choice that required all existing visuals and much of the game's code to be thrown away, and was done without a delay nor much in the way of cel animation knowledge from the team, requiring unpaid overtime. Despite this story's surface-level similarities to crunch practices, the challenge of remaking the game on such short notice was chosen and consented to by the Freddi Fish team, and developers of the game have reminisced on this period fondly.
Kabuto, the first boss of Alex Kidd in Shinobi World, is a parody of Ken-O, a recurring first boss in the Shinobi series - in prerelease screenshots, he is shown to be a composite parody of both Ken-O and Nintendo's Mario, known as Mari-Oh. While his appearance would lose its Mario-esque qualities in the final game, two elements of this parody would remain: Kabuto shooting fireballs, and Kabuto becoming small on his final hit point.
Inside the files of M&M's Kart Racing is a placeholder loading screen image showing what appears to be a 3D render of a 2009 Chevrolet Corvette ZR1 photoshopped over the Red M&M driving his kart copied and pasted over and over erratically.
Inside the files of Game & Watch Gallery 2, there are graphics for an unused Peach version of Modern Ball to go alongside the light blue Yoshi, Mario, Wario, and Bowser versions.
According to Suikosource user JiN88, the Japanese demo of Suikoden II contains two debug/testing rooms.
The first test room claims to be in Radat Town, despite seeming to be in Kyaro Town. The room contains various warping and unfinished textures, out-of-place objects like barrels, and two non-functioning Warp points. Of the warp points, one can be found down near a river and loads nothing, while the other can be found near a bridge with an Anita NPC and some invisible objects. While this test room seems to be empty, except for one small area featuring several invisible objects, a pushable tree that you can talk to, and a row of NPCs that can be pushed, but cannot be interacted with without crashing the game. There are also two Gabocha NPCs, one of which can warp you back to somewhere else in the test room, while the other has some dialogue and acts like a shopkeeper. Finishing the conversation with the shopkeeper Gabocha will cause the game to crash. Interacting with any Anita NPC will cause the game to crash.
The second test room contains several repeating groups of Anita, Gengen, and Old Man NPCs in a sort of army formation. The name of this second debug room is "神様の村" which means "Village of God"/"God's village". Talking to the Anita and Gengen NPC groups will have them say a single line of dialogue (the Anitas however will have no text), and once finished will cause the game to try and load something, which results in the game crashing. It's thought that they would try to warp you somewhere with a cutscene. Raww Le Klueze, global admin of the Suikosource forums, has translated this room's dialogue as such:
"The Gengens each say: • "I am the sound change god" • "I am the sound test god" • "I am the window change god" • "I am the unit change god" (for this phrase, he uses the same word as Apple does when you rearrange war units)
Some of what the Old Man says follows the same format "I am the god of !" in the middle row Library, Suggestion box, Restaurant, Cooking Battles, Guardian Deity and Peeping...? (He also laughs after that one. Bath scenes maybe?)
Top says "The detective god is here!" Bottom "Fishing god here!" "I'm the god of tablets" (same word as the plates found in the Sindar ruins, presumably tests that?)
Last one on the bottom I don't know, he just seems to be making noise cause it just says BABANBABANBANBAN - HAAPIBANONO."
This latter piece of dialogue might be a reference to the song "Nice Hot Water", a 1966 song in the "Nihon no Uta" Japanese local song series that was famously covered by the Japanese comedy rock band The Drifters. The first line before the hyphen is the same as the song, and the second line after the hyphen is almost the same as the song.
When the PlayStation 2 was revealed in Japan, a demo was shown off of a fountain of spark particles. When this demo was shown to Jon Burton, founder of Traveller's Tales, he coded an identical tech demo for the first PlayStation as a joke. This tech demo would ultimately end up in the files of Toy Story 2: Buzz Lightyear to the Rescue!, unused, by accident.
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.
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).
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.
In Garfield's alternate costume texture, there is a portion devoted to Garfello, Garfield's Ninja Turtle alter ego from the Garfield Meets the Teenage Mutant Ninja Turtles short comic published in the 1992 Teenage Mutant Ninja Turtles Magazine.
There is an unused model of the Statue of David by Michelangelo in Squidward's prop set. This was likely intended for his air down strong, where he slams down with a block of marble, referencing the SpongeBob episode "Artist Unknown", where SpongeBob makes the same statue by accident. The model has a clam on its groin as censorship, just like in the original episode, but does not have the nose Squidward places on it.
During development of Pizza Tower, multiple demo builds were released - among these was a build known as the "Peter Griffin Experience", built off of the 2018 "early test build". This demo replaced every single sprite of Peppino with a highly compressed edit of a stock image of Peter Griffin from Family Guy made to resemble Peppino, and replaced one of the game's musical tracks with a fan-made Family Guy remix.
The Case of the Missing Kelp Seeds was the first Humongous Entertainment game to use cel animation instead of computer-designed sprites. This was a last second choice that required all existing visuals and much of the game's code to be thrown away, and was done without a delay nor much in the way of cel animation knowledge from the team, requiring unpaid overtime. Despite this story's surface-level similarities to crunch practices, the challenge of remaking the game on such short notice was chosen and consented to by the Freddi Fish team, and developers of the game have reminisced on this period fondly.
Kabuto, the first boss of Alex Kidd in Shinobi World, is a parody of Ken-O, a recurring first boss in the Shinobi series - in prerelease screenshots, he is shown to be a composite parody of both Ken-O and Nintendo's Mario, known as Mari-Oh. While his appearance would lose its Mario-esque qualities in the final game, two elements of this parody would remain: Kabuto shooting fireballs, and Kabuto becoming small on his final hit point.
Inside the files of M&M's Kart Racing is a placeholder loading screen image showing what appears to be a 3D render of a 2009 Chevrolet Corvette ZR1 photoshopped over the Red M&M driving his kart copied and pasted over and over erratically.
Inside the files of Game & Watch Gallery 2, there are graphics for an unused Peach version of Modern Ball to go alongside the light blue Yoshi, Mario, Wario, and Bowser versions.
According to Suikosource user JiN88, the Japanese demo of Suikoden II contains two debug/testing rooms. 