Within the Nintendo Switch firmware prior to Version 4.0.0 (for Japanese, US and European systems only), there is a hidden NES emulator stub called "flog" that can be unlocked under very specific conditions. flog can only be unlocked on the Home Screen and when the console's internal clock is set to July 11th (if the date is changed in System Settings, but the console is connected to the internet and can see the actual date, this method will not work). The method to unlock it involves detaching the Joy-Cons from the console, holding them pointing forwards/downwards, then moving them to a vertical position and holding it for a few seconds. This gesture may take some time to hone due to it being a specific movement tracked by the Joy-Cons, but when it is matched, the system will check to see if flog is installed. When checked, an audio clip of a man saying "chokusetsu" ("直接"), the Japanese word for "direct", will play and the screen will cut to black and launch the 1984 NES title Golf. This emulator is unique in that it includes specific instructions in English and Japanese on how to play depending on how the Joy-Cons are held, and has a more stripped-down and simplistic appearance than the emulators that would be used for NES games on Nintendo Switch Online. Pressing the Home button while playing Golf will return you to the Home Screen without any visible software running there.
With Version 4.0.0, Nintendo removed all of the code required to launch flog and play Golf, but the company seemed unusually hesitant to even acknowledge its existence when asked by news outlets. One month before its removal, Nintendo of America president Reggie Fils-Amie released two statements regarding it when asked by Kotaku:
"So, two comments on this. It was identified by folks playing around in the firmware. We've got nothing officially to announce for that content or what the plans are for that content. So that's that. Certainly anything that pays respect to my friend Mr. Iwata is something that is near and dear to me personally, but in terms of that execution and what it was meant to do or what the plans are, we've got nothing to announce."
"I'm struck whenever I go back to Kyoto and spend time in our headquarters and spend time in the offices where Mr. Iwata, myself and others would be meeting. It's always personally touching. And so, again, no comment on that particular execution."
While this seems to allude that the secret emulator and Golf's inclusion were not authorized within Nintendo, this all but confirms that their purpose was to act as a tribute to Nintendo's late CEO Satoru Iwata. Iwata, who programmed Golf and previously hosted the company's Nintendo Direct showcase series, passed away on July 11th, 2015, with the method to unlock the emulator mimicking a gesture he used during Nintendo Directs. Japanese fans on social media referred to the Easter egg as an "omamori", an amulet purchased at Shinto shrines and Buddhist temples in Japan that if kept close are said to protect the bearer and bring good luck, speculating that Golf was included by Iwata as a secret charm to watch over every Nintendo Switch unit after his death.
In PAL releases of the PlayStation 2 version, the player is normally only able to access the tutorial in English, French, Italian, Castilian, and Dutch. Despite this, versions in the other supported languages (German, Swedish, Finnish, Danish, Norwegian, and Portuguese) are present in the game's data and can be accessed either through hacking the game or selecting a supported language and switching to an unsupported one with debug features just before the screen fades to black. It is unknown why these six variants of the tutorial were made inaccessible in the final game, given that they are fully translated.
The North American arcade release of Double Dragon 3: The Rosetta Stone is believed to be the first game to incorporate microtransactions, allowing players to purchase extra lives, characters, energy and moves by putting more quarters into the arcade cabinet. This would be removed for the Japanese release, and replaced with a conventional character select screen that lets you play as the previously paywalled extra characters, and the game's difficulty was rebalanced to be easier to adjust.
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.
Normally, when the player decides to build a public works project, Isabelle will prevent the player from going to the beach while scouting out a potential site due to the instability of the sand there. However, if the player uses cheats or glitches to bring Isabelle to the beach and then talks to her, she will give them the option to state whether or not they want to start construction. Despite this, she will deny the request anyway if the player answers yes; however, she will not cite the quality of the ground, but rather will state that there is no available space. Depending on where they are standing, she will state that the proposed site is either too close to the ocean or too close to the cliff, statements which do not otherwise come up in normal gameplay. This dialogue implies that building public works projects on the beach was considered at one point in development.
At some point, forest fires were one of the many disasters that could occur in the player's parks. There was also a Sprinkler System that was planned as an upgrade for structures in-game that would have been used to mitigate the aforementioned forest fires. All of this would be removed in the final game.
There is an unused death cutscene in the files of Bubsy 3D of Bubsy falling through a floor. This may have been an animation for falling through pits, or suggest that fall-damage was at one point going to return as a mechanic from Bubsy in Claws Encounters of the Furred Kind.
In the seventh episode of each course, the player is required to chase down and defeat Shadow Mario to obtain a Shine Sprite, which spawns from his dematerializing body as he teleports away. In actuality, the Shine Sprites are stored at fixed locations and are simply teleported to his location by the flag "appearShineFromKageMario". If the flag is replaced with "appearShineFromNPC", the Shine Sprites appear where they are stored, implying that these were their originally intended locations. The fixed points in "Shadow Mario Revisited", "Hold It, Shadow Mario!", and "Shadow Mario Runs Wild" also feature dedicated cameras for spawning cutscenes, corroborating the idea.
Of note is that the internal location for the Shine Sprite in "Shadow Mario Runs Wild" places it partially inside Pianta Village's golden mushroom, which further implies that this object had its height raised at some point during development.
Within the data for Pokémon Diamond, Pearl, and Platinum is code to spawn in a Level 40 Darkrai as a roaming Pokémon, tying in with its nature as a malevolent counterpart to Cresselia (which similarly becomes a roaming Pokémon after meeting the player on Fullmoon Island). In the final game, Darkrai is instead encountered as a stationary opponent at Newmoon Island after obtaining the Member Card as a Mystery Gift item at select in-person events.
In early February 2024, one month prior to the game's release, multi-time WWE World Champion Brock Lesnar was removed from the cover of the "Forty Years of WrestleMania" showcase edition of the game in response to a sexual assault and sex trafficking lawsuit against Vince McMahon and WWE. Although Lesnar's name was not directly mentioned in the lawsuit, his reported connection to the case led to his removal as part of a wider effort by the WWE to disassociate from him. The cover was updated to feature a larger version of John Cena, where Lesnar used to be.
Content creators who got early access to the game on February 23 confirmed that both Lesnar and McMahon were no longer playable characters in WWE 2K24. While the extent of McMahon's removal is unknown, Brock Lesnar’s data remains locked away in the game due to his matches with The Undertaker at Wrestlemania 30 and Roman Reigns at Wrestlemania 31 being featured in the "Showcase of the Immortals" Showcase mode, and it being too far into development to remove them.
An unused cutscene in the game has the character Stumbler O'Hare openly mock Harley Mullins (the wife of Mark Mullins) and her situation. This was scrapped both for being "WAY too on the nose", and because Stumbler's personality was altered during development. The cutscene would be replaced with a YouTube video titled "How to say Goodbye", in which Stumbler, despite knowing about Mark's crimes, tries to help him grieve over the loss of Harley. Despite the cutscene's removal, Stumbler's statue in The Dam still has him wearing a nurse's cap like in the cutscene.
The concept of Mario riding a dinosaur was first concieved for Super Mario Bros., but was considered impossible from a technical standpoint. Eventually, this concept would be used in Super Mario World with Yoshi.
Atari 2600's Asteroids has an unused functionality for showing graphics of the spaceship as a life counter instead of the number, much like the arcade version. This was most likely changed for processing power reasons, as only six ships can be displayed at a time.
Earthworm Jim: Menace 2 the Galaxy has unused text pertaining to Game Boy Printer support. It's thought that this feature would have allowed players to print out level passwords.
The E3 2003 reveal trailer for what would become Star Fox: Assault showcased what appears to be a first-person perspective mode, which ended up being discarded in the final game. However, this scrapped first-person mode can still be accessed in all versions of the game through hacking. It is not fully complete, but has almost full functionality.
3 items from Nickelodeon All-Star Brawl were removed before the items update: a fart cloud, the Krusty Krab blimp, and a wrench set. The fart cloud and blimp's use are not fully known, but the wrench set would've created a platform like Toph's down special. They were rejected "because they weren't that fun".
In the files of the game, there is content implying two other potential appearances for the Krusty Krab Blimp: a texture for the Kids' Choice Awards blimp, and voice lines from the announcer referencing the Teenage Mutant Ninja Turtles' blimp, which is used in April's up special, as an item.
In the files of the Donkey Kong Adventure expansion, there are unused files for Rambi the Rhino as a party member, including a model; menu icons; and skills. He would have taken up two spaces on the board and been able to charge through any destructible terrain that enemies are using as cover and destroy it instantly. In the final DLC, Rambi would not appear physically in any form, but would have a gun made in his image. Rambi's model looks closer to his appearance in Rare's Donkey Kong games than his appearance in the Paon and Retro Studios Donkey Kong games, a design change that would also be carried over to Super Nintendo World's iteration of the character after Donkey Kong Adventure's release.
The matching card game has an unused card called "MatchPointsSadChef". It does not have any attached graphics. While this card doesn't exist, the game does have a happy chef card, using the chef from Comfy Cakes.
South Park: Chef's Luv Shack has an unused minigame called WBALL, where the boys would throw water balloons at passerby from a tall building. It exists in the PlayStation version and a prototype of the Dreamcast version, but it is not currently known to be playable.
The North American arcade release of Double Dragon 3: The Rosetta Stone is believed to be the first game to incorporate microtransactions, allowing players to purchase extra lives, characters, energy and moves by putting more quarters into the arcade cabinet. This would be removed for the Japanese release, and replaced with a conventional character select screen that lets you play as the previously paywalled extra characters, and the game's difficulty was rebalanced to be easier to adjust.
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.
At some point, forest fires were one of the many disasters that could occur in the player's parks. There was also a Sprinkler System that was planned as an upgrade for structures in-game that would have been used to mitigate the aforementioned forest fires. All of this would be removed in the final game.
In early February 2024, one month prior to the game's release, multi-time WWE World Champion Brock Lesnar was removed from the cover of the "Forty Years of WrestleMania" showcase edition of the game in response to a sexual assault and sex trafficking lawsuit against Vince McMahon and WWE. Although Lesnar's name was not directly mentioned in the lawsuit, his reported connection to the case led to his removal as part of a wider effort by the WWE to disassociate from him. The cover was updated to feature a larger version of John Cena, where Lesnar used to be. 
The concept of Mario riding a dinosaur was first concieved for Super Mario Bros., but was considered impossible from a technical standpoint. Eventually, this concept would be used in Super Mario World with Yoshi.
Atari 2600's Asteroids has an unused functionality for showing graphics of the spaceship as a life counter instead of the number, much like the arcade version. This was most likely changed for processing power reasons, as only six ships can be displayed at a time.
The E3 2003 reveal trailer for what would become Star Fox: Assault showcased what appears to be a first-person perspective mode, which ended up being discarded in the final game. However, this scrapped first-person mode can still be accessed in all versions of the game through hacking. It is not fully complete, but has almost full functionality.
3 items from Nickelodeon All-Star Brawl were removed before the items update: a fart cloud, the Krusty Krab blimp, and a wrench set. The fart cloud and blimp's use are not fully known, but the wrench set would've created a platform like Toph's down special. They were rejected "because they weren't that fun".
In the files of the Donkey Kong Adventure expansion, there are unused files for Rambi the Rhino as a party member, including a model; menu icons; and skills. He would have taken up two spaces on the board and been able to charge through any destructible terrain that enemies are using as cover and destroy it instantly. In the final DLC, Rambi would not appear physically in any form, but would have a gun made in his image. Rambi's model looks closer to his appearance in Rare's Donkey Kong games than his appearance in the Paon and Retro Studios Donkey Kong games, a design change that would also be carried over to Super Nintendo World's iteration of the character after Donkey Kong Adventure's release.
