There exists an extremely early test prototype of Disney's Lilo & Stitch 2: Hänsterviel Havoc that is simply just a compilation of a platforming level from Ed, Edd n' Eddy: Jawbreakers and a racing level from Hot Wheels: World Race as a proof-of-concept that both styles of gameplay could fit in a single game pak. The only element of Lilo & Stitch that exists with in the game is Eddy's sprite in the Jawbreakers segment being replaced with Stitch.
According to the British magazine Nintendo Magazine System, before settling on Yoshi's Island characters for Tetris Attack, Nintendo reportedly asked Rare to replace Panel de Pon's fairies with characters from the Killer Instinct series.
According to an Electronic Gaming Monthly preview article, Cool Spot was originally a Reversi clone like Spot: The Video Game, titled Spot II - the graphics used in Cool Spot seem to have been reused from cutscenes in the cancelled game.
The Madden curse was an urban legend which claimed that athletes who appeared on the box of Madden games would either under-perform or sustain an injury in the season following their game's release. After a 16-year streak of "bad luck" starting from Garrison Hearst's appearance on the box of Madden NFL 99 in 1998, the first Madden game to feature a cover athlete and not John Madden himself, the "curse" would finally be broken in 2014, when Calvin Johnson managed to have one of the best seasons in his career following his appearance on the box of Madden NFL 15. Although the streak ended, the curse would strike sporadically over the continuning years, but so far has not begun another continuing streak of bad luck.
In 2010, a film adaptation of the Madden curse was announced by EA, but no news has surfaced on it since and it can be assumed cancelled.
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.
In the lead-up to the release of Super Villain Island, a piece of concept art titled "Combo Breaker" was posted on the Daily Pop, depicting what appears to be a robot that combined elements from the four villains featured in the island's story (rabbit traits from Dr. Hare, jester traits from Mordred/Binary Bard, artist traits from Black Widow, and pirate traits from Captain Crawfish), as well as what appears to be traits reminiscent of a cowboy. This, combined with an earlier Daily Pop post titled "Perchance to Dream" that suggested the Dream Machine was originally going to have five containment units instead of four, has lead some fans to believe that El Mustachio Grande (the main antagonist of Wild West Island) was originally going to appear alongside the former four as one of the main villains. While this has never been confirmed, it's worth noting that El Mustachio Grande appeared as one of the answers to the Pop Quiz question "Who is your favorite Poptropica villain?" alongside the four. Additionally, while he does not play a role in the story of Super Villain Island, he does still make a cameo during the elevator sequence alongside various other Poptropica villains.
Bio Force Ape is a game that was never released - it did, before it's cancellation, get a spotlight in Nintendo Power, creating a level of curiosity surrounding it within NES fan communities. Capitalizing off of this curiosity, a post would be made on the Digital Press forum in 2005 claiming to show screenshots of a leaked prototype. While the first post appeared legitimate, the hoax would eventually be unraveled starting with a screenshot of a glitched super move that was "so powerful [that] it messes up the game's graphics", which humorously made it appear that Bio Force Ape was unleashing a powerful fart attack, as a set of glitched graphics appeared next to a crouching animation. The poster, going under the username PaulB812, would refuse to dump the game and refer to anybody who asked for it to be released as either "communists" or "butter-slathered... hoarding fatties", before finally unveiling the prank with a game screenshot of a cutscene where a fat butter monster points out that the ape is "worth 2K monies[sic]", before the ape punches him while saying "EAT COMMUNISM!" A real prototype would be leaked in 2010.
In 2013, around the launch of Disney Infinity, Disney merchandise artist Jim Valeri posted a set of images to his portfolio of angular figures, resembling the character models from Disney Infinity, of Muppets Kermit, Gonzo, Miss Piggy, Animal, Beaker, and Fozzie Bear, mentioning that they were made a year ago and he had no clue what they were for or if they would be released. While not explicitly stated, it seems heavily likely from the timing and aesthetic that these were intended as characters for Disney Infinity who never saw the light of day, possibly due to the financial failure of the 2014 film Muppets Most Wanted.
The HVC Controller Test, a cartridge used internally at Nintendo to test Famicom controllers, contains a hidden Simon-style game, accessed by pressing B and Down on Controller 2.
The reptilian puffs of smoke that emerge when Mario ground pounds inside a giant Goomba Shoe in the Super Mario Bros. 3 style of the Super Mario Maker series are updated sprites from an unreleased and highly experimental Super Nintendo prototype known as Super Donkey.
Unusually, the 3DS version of Donkey Kong: Original Edition was originally released with an unfinished ROM predating the hack's previous Wii release. The only change in the unfinished version was that the copyright assigned the game as a 2008 release instead of 2010, suggesting that it was originally intended to be released for the 25th anniversary of Donkey Kong's Famicom port and delayed for Super Mario Bros.' anniversary. The game would soon be patched the use the Wii ROM instead.
At some point in the 2000s, a promo video was made by THQ and Toys R Us to encourage Toys R Us store owners to stock THQ video games. This tape opens with a strange, off-model rendition of SpongeBob not resembling any of his video game models and with notable details such as his pore locations, finger shape, and amount of fingers being completely off from Nickelodeon's style guide. The video also has imitation music not taken from either the SpongeBob SquarePants TV show nor any of its video games. Despite this, the video does use legitimate background art from the cartoon, and has voice work seemingly done by SpongeBob's voice actor Tom Kenny. While SpongeBob's model seems to be original, Patrick's model is taken from Nicktoons Unite!, sporting the distinctive face-long mouth and half-shut eyelids of his model used throughout the Nicktoons platformer game series. The complete promo video is not known to have been preserved.
In four separate 1989 issues of Video Games & Computer Entertainment, the mail-in game order service Play It Again put up an advertisement featuring a list of games customers could order. Among these is a mysterious title, Yeah Yeah Beebiss 1, which is not the name of any one game - this title would reappear in a separate advert for FuncoLand, under the name "Yeah Beebiss 1". To add further mystery to this game, it was placed between W and X games alphabetically instead of with Y games. There are multiple theories on the status of Beebiss:
•Some believe that Beebiss is a baby-themed game like Baby Boomer, Rai Rai Kyonshis: Baby Kyonshi no Amida Daibouken, or Bio Miracle Bokutte Upa, given that "baby" sounds like "Beebiss" - with Rai Rai Kyonshis being considered the most likely candidate of the 3 for "Rai Rai"'s (A Japanese onomatopoeia associated with China) similarity to "Yeah Yeah", the game's release in 1989, and the presence of other import games on the listing. Rai Rai Kyonshis is generally the most accepted identity of the game. •One theory proposes that the game was made as a copyright trap to catch which retailers are stealing game lists, something one of the founders of Play It Again confessed to doing, albeit without confirming if Beebiss was among those games. •A rather strange theory proposes that the game was somehow related to exploring, possibly being Atlantis no Nazo's cancelled US release Super Pitfall II, named for Charles William Beebe, a famous explorer and naturalist •Another theory suggests that the title was simply a bizarre, elaborate in-joke that both Play It Again and FuncoLand employees were in on.
The myth of Beebiss would inspire the game Yeah Yeah Beebiss II, developed by popular retro gaming YouTuber John Riggs and based on the aesthetic of Rai Rai Kyonshis.
When Elite Software obtained the license to make a video game based on the ThunderCats cartoon, they were unsure if they would be able to complete the game in time for Christmas, and as such outsourced a second ThunderCats game to Paradise Software. Neither game could be finished before the deadline, and as such Elite instead attained the rights to a near-finished game titled Samurai Dawn by FTL Games and released it as ThunderCats: The Lost Eye of Thundera.
Elite's in-house ThunderCats game got pitched to Capcom as a sequel to Ghosts 'n Goblins, but was not picked up due to Ghouls 'n Ghosts being in development at the time. It would be released as an original IP, simply titled Beyond the Ice Palace, featuring a character who in certain versions of the game resembles Lion-O.
Paradise Software's ThunderCats game remains shrouded in mystery, but it seems incredibly likely that the game was released as Bomb Jack II, owing to the C64 version of the game including a rendition of the ThunderCats theme song (stolen music being somewhat of a tradition for the Bomb Jack franchise), not featuring any bombs, and having a protagonist that more closely resembles Lion-O than Bomb Jack.
An easier, medical version of Fruit Ninja was created in collaboration with Neuroscience Australia for stroke rehabilitation programs in hospitals; this version of the game could export data into patients' medical records.
Across two interviews with two different Nintendo Dream issues in 2011, Takaya Imamura expressed quite a bit of interest in creating a prequel to Star Fox 64 starring James McCloud and the original Star Fox team, mentioning the idea three times, unprovoked. He also even suggested a future game could be a soft reboot set several years later with a new lead instead of Fox.
From the Star Fox 64 3D Developer interview in the September 2011 issue of Nintendo Dream:
Why did Pigma join the original Star Fox team?
Imamura: I still don't want to talk about the events surrounding James or other episodes from the original Star Fox because if we were to do something with that, I would want to create a game around it and release it.
Ohhhh!
Imamura: I'm just thinking out loud.
Everyone: (laughs)
Imamura: You see, if we talk about it too much here, it might become difficult to make if it really happens (laughs).
Shortly aftwards, after being asked about the future of the Star Fox series:
When Nintendo makes a video game, we don't make it to tell a story. We first have some kind of cool idea for the core of a game, and if that idea is something that works well for Star Fox, then we need to build a story around it. When that time comes, it might be a story about James, or we might just keep the "Star Fox" franchise name and set it far off in the future with a different main character.
But really, part of me does want to end Fox's part of the story with "Command." So, going forward, if we made a sequel it might be set between "64" and "Adventures," or maybe even a prequel to "64."
Unfortunately, nothing has ever come of this. All the Star Fox series has seen since then is the release of Star Fox Zero (a reimagining of Star Fox 64), Star Fox Guard (a tower defense game set around the same time), and an official release of the 20-year old game Star Fox 2. Takaya Imamura would retire from Nintendo in early 2021.
Vib-Ribbon was not initially released in the US due to Sony Computer Entertainment America being unimpressed by the game's simplistic graphics. Designer Masaya Matsuura encouraged fans to campaign for the game's US release, but this was seemingly unsuccessful as the game did not see a release there until the PlayStation 3 in 2014.
In the early 1980s, 20th Century Fox's game division planned to release a trio of environmentalist games designed by Steve Beck - Save the Whales, Dutch Elm Defender, and Attack of the Baby Seals - to raise money for various charities and organizations. None of them were released, and Save the Whales is both the only one with a known attached charity (Greenpeace) and the only one that ever entered development.
In support of the New Game+ dungeon and Sindar Ruin connection, Suikosource user and dataminer JiN88 explored the Japanese demo to support the idea that Sindar Ruin underwent some major changes during development:
• In the entrance between White Deer Inn and Sindar Ruin, there are 2 statues that are generic. In the demo, those statues are gendered with pronounced breasts, and appear snake-like, resembling the Greek mythical monster Lamia. The generic statues exist in the demo, so there is no reason the developers would go out their way to make a different pair.
• At the last puzzle where you put the plate in, the plaque that you would normally read is there instead of in the room next ahead.
• The two statues of the Double Head are absent.
• In the boss room of Sindar Ruin, there is a weird sprite that isn't seen anywhere else (it's a bit grainy so to see it you will need to zoom in). The position looks like it's supposed to be for an arch, but notably it has three heads. Double Head is the boss you would normally meet here only with two heads, suggesting there was originally a boss here with three heads.
There exists an extremely early test prototype of Disney's Lilo & Stitch 2: Hänsterviel Havoc that is simply just a compilation of a platforming level from Ed, Edd n' Eddy: Jawbreakers and a racing level from Hot Wheels: World Race as a proof-of-concept that both styles of gameplay could fit in a single game pak. The only element of Lilo & Stitch that exists with in the game is Eddy's sprite in the Jawbreakers segment being replaced with Stitch.
According to an Electronic Gaming Monthly preview article, Cool Spot was originally a Reversi clone like Spot: The Video Game, titled Spot II - the graphics used in Cool Spot seem to have been reused from cutscenes in the cancelled game.
In the lead-up to the release of Super Villain Island, a piece of concept art titled "Combo Breaker" was posted on the Daily Pop, depicting what appears to be a robot that combined elements from the four villains featured in the island's story (rabbit traits from Dr. Hare, jester traits from Mordred/Binary Bard, artist traits from Black Widow, and pirate traits from Captain Crawfish), as well as what appears to be traits reminiscent of a cowboy. This, combined with an earlier Daily Pop post titled "Perchance to Dream" that suggested the Dream Machine was originally going to have five containment units instead of four, has lead some fans to believe that El Mustachio Grande (the main antagonist of Wild West Island) was originally going to appear alongside the former four as one of the main villains. While this has never been confirmed, it's worth noting that El Mustachio Grande appeared as one of the answers to the Pop Quiz question "Who is your favorite Poptropica villain?" alongside the four. Additionally, while he does not play a role in the story of Super Villain Island, he does still make a cameo during the elevator sequence alongside various other Poptropica villains.
Bio Force Ape is a game that was never released - it did, before it's cancellation, get a spotlight in Nintendo Power, creating a level of curiosity surrounding it within NES fan communities. Capitalizing off of this curiosity, a post would be made on the Digital Press forum in 2005 claiming to show screenshots of a leaked prototype. While the first post appeared legitimate, the hoax would eventually be unraveled starting with a screenshot of a glitched super move that was "so powerful [that] it messes up the game's graphics", which humorously made it appear that Bio Force Ape was unleashing a powerful fart attack, as a set of glitched graphics appeared next to a crouching animation. The poster, going under the username PaulB812, would refuse to dump the game and refer to anybody who asked for it to be released as either "communists" or "butter-slathered... hoarding fatties", before finally unveiling the prank with a game screenshot of a cutscene where a fat butter monster points out that the ape is "worth 2K monies[sic]", before the ape punches him while saying "EAT COMMUNISM!" A real prototype would be leaked in 2010.
In 2013, around the launch of Disney Infinity, Disney merchandise artist Jim Valeri posted a set of images to his portfolio of angular figures, resembling the character models from Disney Infinity, of Muppets Kermit, Gonzo, Miss Piggy, Animal, Beaker, and Fozzie Bear, mentioning that they were made a year ago and he had no clue what they were for or if they would be released. While not explicitly stated, it seems heavily likely from the timing and aesthetic that these were intended as characters for Disney Infinity who never saw the light of day, possibly due to the financial failure of the 2014 film Muppets Most Wanted.
The SNES burn-in test cartridge used internally for repairs at Nintendo contains a 16-bit rendition of the theme to Zelda II: The Adventure of Link.
The reptilian puffs of smoke that emerge when Mario ground pounds inside a giant Goomba Shoe in the Super Mario Bros. 3 style of the Super Mario Maker series are updated sprites from an unreleased and highly experimental Super Nintendo prototype known as Super Donkey.
Unusually, the 3DS version of Donkey Kong: Original Edition was originally released with an unfinished ROM predating the hack's previous Wii release. The only change in the unfinished version was that the copyright assigned the game as a 2008 release instead of 2010, suggesting that it was originally intended to be released for the 25th anniversary of Donkey Kong's Famicom port and delayed for Super Mario Bros.' anniversary. The game would soon be patched the use the Wii ROM instead.
At some point in the 2000s, a promo video was made by THQ and Toys R Us to encourage Toys R Us store owners to stock THQ video games. This tape opens with a strange, off-model rendition of SpongeBob not resembling any of his video game models and with notable details such as his pore locations, finger shape, and amount of fingers being completely off from Nickelodeon's style guide. The video also has imitation music not taken from either the SpongeBob SquarePants TV show nor any of its video games. Despite this, the video does use legitimate background art from the cartoon, and has voice work seemingly done by SpongeBob's voice actor Tom Kenny. While SpongeBob's model seems to be original, Patrick's model is taken from Nicktoons Unite!, sporting the distinctive face-long mouth and half-shut eyelids of his model used throughout the Nicktoons platformer game series. The complete promo video is not known to have been preserved.
In four separate 1989 issues of Video Games & Computer Entertainment, the mail-in game order service Play It Again put up an advertisement featuring a list of games customers could order. Among these is a mysterious title, Yeah Yeah Beebiss 1, which is not the name of any one game - this title would reappear in a separate advert for FuncoLand, under the name "Yeah Beebiss 1". To add further mystery to this game, it was placed between W and X games alphabetically instead of with Y games. There are multiple theories on the status of Beebiss:
Across two interviews with two different Nintendo Dream issues in 2011, Takaya Imamura expressed quite a bit of interest in creating a prequel to Star Fox 64 starring James McCloud and the original Star Fox team, mentioning the idea three times, unprovoked. He also even suggested a future game could be a soft reboot set several years later with a new lead instead of Fox. 
In support of the New Game+ dungeon and Sindar Ruin connection, Suikosource user and dataminer JiN88 explored the Japanese demo to support the idea that Sindar Ruin underwent some major changes during development: