Inside the files of SpongeBob SquarePants: Battle for Bikini Bottom, you can find a video from Scooby-Doo! Night of 100 Frights, an earlier game by the same studio, Heavy Iron. It is unknown if this is a leftover from building Bikini Bottom off of 100 Frights' engine, or a cross-promotion that was veto'd by either Warner Bros. or Nickelodeon.
What some may find more interesting, however, is a now-deleted YouTube upload of the unused cutscene colloquially known as the "Cursed YouTube Video" which was based on a corrupted video file and as such featured disorientating, lagging sound; an inaccurate timer for video length; visuals that cut out; and a buffer that would begin at an exact point and continually play regardless of the viewer's internet speed.
Expresso the Ostrich from Donkey Kong Country appears as a cameo in Banjo-Pilot - this seems to be an accidental leftover from the game's cancelled iteration as Diddy Kong Pilot, as Nintendo is not mentioned whatsoever in the copyright info for Banjo-Pilot beyond simply licensing the game for its platforms.
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 the Demo ending, other than the dialogue used in the skit, there is unused dialogue present suggesting some parts of the ending were scrapped:
”I think that‘s a wise decision.” ”The normal ending is about to start.” ”The best ending is about to start.” ”You‘re wrong, brother Mukumuku. I heard that it‘s not done yet.” ”Erp.....”
It's worth noting that Mukumuku is silent in the demo while Mekumeku just doesn't load, but is present in memory. Whether it was scrapped or is still unlockable is unknown.
Also in the room, there are 4 maids surrounding Riou just out of bounds at the bottom of the screen. After some testing, it was found to be a developer trick, wherein the player is always in control of Riou, and when interreacting with any of the maids, the demo will end early.
Hades was originally planned to be the final boss as evidenced by early concept art of the Hill of Despair showing your party facing Hades in the area instead of Necron, but Hades was instead changed to be an optional boss. There are eyes on the face of his sword and on his throne. Blinking disembodied eyes are also seen in the final battle arena, showing the connection between Hades and the original final battle. Incidentally, eyes are also a feature of many Terran structures (e.g. Pandemonium), perhaps implying a deeper connection that was lost or severed in the final cut of the game.
While no exact reason for the change was given, it is believed that Hades ultimately did not fit the theme as an end boss. Hades is based on Greek mythology as a God of death who would have wanted to bring death to everyone. Necron on the other hand is an original entity based on the Greek word for "death" that wanted to bring an end to existence itself.
The DS version of Hotel Transylvania contains an early version of Lalaloopsy: Carnival of Friends' credits - this notably includes a joke title for WayForward president Voldi Way "Tyrannical Overlord". This file was removed in the European release of the game.
The Nintendo 64 and PlayStation versions of Tigger's Honey Hunt have an unusal cheat list in their data, seemingly related to another unknown game.
Cheats Hero Unlimited Live : On Hero Unlimited Live : Off Enemy Unlimited Live : On Enemy Unlimited Live : Off Unlimited Power Up : On Unlimited Power Up : Off Display Hero Live : On Display Hero Live : Off Unlimited Continue : On Unlimited Continue : Off End RTC : On End RTC : Off Test Fatality : On Test Fatality : Off Kill Enemy : On Kill Enemy : Off _ Previous Menu Combo |,},}.Combo },|,|.Combo {,},}.Combo },{,{.Fatality }+|
In a developer interview published in the March 1999 issue of DenGeki PlayStation magazine, it was revealed that Rowd, a character who betrayed the Unicorn Youth Brigade of the Highland Kingdom, was the older brother of the little blind girl in Kyaro Town who disappears near the end of the game, and his motivation for his betrayal was to save more money to cure her blindless. When talking to the girl in Kyaro Town, she states that her big brother plans to use his riches to hire a famous doctor in the Toran Republic (who is suspected to be Liukan, a character from Suikoden who briefly appears in this game) to perform the operation. During the interview, programmer Akiyoshi Ohta also stated that there was originally going to be a battle against Rowd where he would die, but this was cut because "his little sister was too cute to do it."
In the Japanese dub of the game, during the December 4th visit to the hospital to question Taro Namatame, he says the line: Spoiler:"Then… little by little, the image came into sharper focus." After the end of this line, you can very faintly hear a click followed by what sounds like the rustling of paper. This audio appears to be Namatame's voice actor Yasuhiro Roppongi turning the pages of his script after finishing his line read, and was accidentally left in the final game.
Sonic Origins contains leftover data from the then-unreleased Sonic Frontiers. This is how it was first discovered that Big the Cat and his fishing minigame would return in Frontiers.
As of the current updated version of the game, a walkway overlooking a pier found in the southwest portion of the Financial District region contains a spot that does not have collision detection. This spot makes players fall under the world's map where they can unofficially travel under New York City. By carefully traveling northward towards Greenwich, players will come across leftover assets for an underground structure with pipes that can be web swing towards. Entering this structure and climbing up will reveal a Spoiler: leftover unused Kraven trophy room featuring a throne and the mounted heads of Rhino, Vulture, Electro, & Shocker. This Spoiler:trophy room probably would have coincided with the game's story that Spoiler: Kraven The Hunter kills Scorpion on screen, while Vulture, Electro, & Shocker are killed off screen.
Dr. Starline the Platypus from the IDW Sonic the Hedgehog comic series is inspired by a glitch character from Knuckles' Chaotix - a silver, red and charcoal Knuckles recolor, known by fans as "Wechnia", but by the game itself as "**********", seemingly based on unused data for Tails. Starline's color scheme is taken directly from Wechnia, his name is a literal take on the name "**********", (or "star-line") the black streak in his fur resembles the red streaks all over Wechnia's fur, and the choice of a platypus seems to be inspired by how certain Wechnia sprites would make the bottom of his muzzle yellow and loosely resemble shading for the underside of a flat bill, alongside platypi being Australian like echidnas.
In the Japanese version of Star Fox Adventures, some of the dinosaur tribes have different localized names from their original names. For example, the CloudRunners are referred to as the クラウド族 (Kuraudo-zoku), which translates to "Cloud Tribe". The EarthWalkers are referred to as the アソーカ族 (Asōka-zoku), which strangely seems to translates to "Ahsoka Tribe". "Ahsoka" is a name of Sanskrit origin that means "pure and hopeful".
In Star Fox: Assault's Sauria mission, Peppy curiously refers to the CloudRunner Tribe as just "the Cloud Tribe". This makes more sense when you factor in that that's what they're called in the Japanese version of Star Fox Adventures and thus was what the original script writers for Assault in Japan used as a frame of reference, and it appears that whoever was localizing the script for western audiences didn't bother to change it back to the original Rare name.
Funny enough, in Star Fox Command, Krystal pilots a spacecraft that is both shaped and named after the CloudRunners. In the Japanese version, it actually retains that name, クラウドランナー (Kuraudoran'nā), which means that Command marks the first time Rare's original CloudRunner name was utilized in the Japanese version of these games.
Out of the 4 main survivors, Zoey was the only one to change voice actors, changing from Alésia Glidewell to Jen Taylor. Despite this, almost all of the pain and death sounds you hear in the game are actually lines leftover from Alésia and not Jen. It's unknown if this was intentional as she is not credited in Left 4 Dead.
Star Fox Adventures began development as Dinosaur Planet, which originally began development as a direct sequel to Diddy Kong Racing, starring Timber the Tiger (who himself was intended as the main character of DKR when it was Pro-Am 64, before Donkey Kong characters were incorporated into it). It was pitched as an adventure/racing hybrid where Timber, equipped with a rucksack, little fingerless gloves, and a baseball cap would go back in time and team up with a younger version of Tricky, a boss from the first DKR. Wizpig, the main antagonist and final boss of the first Diddy Kong Racing, was also involved. It would seem a working title for this iteration of the game was "Dino Island", going by several posts on former Rare employee Ed Bryan's Twitter account.
Eventually it was decided to rework the concept into an entirely new original IP with no story connections to DKR and two new main characters, named Sabre and Krystal, and also make it a full-on action-adventure game directly inspired by The Legend of Zelda: Ocarina of Time. DP/SFA's various racing segments are said to be remnants of the original DKR successor concept, and while he retained the name of the DKR boss (something Rare seems to regret), Tricky would wind up becoming his own distinct character. Several files within DP/SFA's data also mention Timber, obviously a remnant from when he was the star.
Sabre and Krystal were in turn eventually replaced by Fox McCloud as the main playable character when the game was rebranded into what we know today as Star Fox Adventures, though Tricky would remain as his sidekick and Krystal would remain as a main character.
Sonic the Hedgehog 4 was originally going to be a mobile exclusive game under the title "Sonic the Portable" before being made into a cross platform game marketed as a sequel to the classic trilogy on the Genesis. This name remains left in the game in Carnival Street Zone where it is displayed on the ferris wheel in the level background on a blurred out neon sign.
During the prerelease of Nickelodeon All-Star Brawl 2, a set of placeholder pages were found on the game's website relating to different characters. Most notable of these pages was one for Helga, a character who was in the first Nickelodeon All-Star Brawl and hadn't been seen in All-Star Brawl 2 marketing materials or leaks at that point - this led a lot of fans to believe that Helga would be returning. Ultimately, she would only be a background cameo on the Aquarium stage.
The 3DS version of Imagine Babyz has unused content pertaining to what seems to be an unreleased 2D shoot 'em up titled Sound Rider. There is a title screen, multiple ship and bullet sprites, and two MOD tracker-styled songs.
Alongside Garfield, Rocksteady from Teenage Mutant Ninja Turtles was originally going to be one of the two post-launch playable fighters in Nickelodeon All-Star Brawl - notably without his partner-in-crime Bebop, much like Michelangelo and Leonardo's inclusion without Raphael and Donatello. He was ultimately replaced by Shredder, and all that remained of his inclusion at launch was a placeholder stock image and the codename "hunter".
Rocksteady would eventually become playable in Nickelodeon All-Star Brawl 2.
Expresso the Ostrich from Donkey Kong Country appears as a cameo in Banjo-Pilot - this seems to be an accidental leftover from the game's cancelled iteration as Diddy Kong Pilot, as Nintendo is not mentioned whatsoever in the copyright info for Banjo-Pilot beyond simply licensing the game for its platforms.
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 the Demo ending, other than the dialogue used in the skit, there is unused dialogue present suggesting some parts of the ending were scrapped:
Hades was originally planned to be the final boss as evidenced by early concept art of the Hill of Despair showing your party facing Hades in the area instead of Necron, but Hades was instead changed to be an optional boss. There are eyes on the face of his sword and on his throne. Blinking disembodied eyes are also seen in the final battle arena, showing the connection between Hades and the original final battle. Incidentally, eyes are also a feature of many Terran structures (e.g. Pandemonium), perhaps implying a deeper connection that was lost or severed in the final cut of the game.
Dr. Starline the Platypus from the IDW Sonic the Hedgehog comic series is inspired by a glitch character from Knuckles' Chaotix - a silver, red and charcoal Knuckles recolor, known by fans as "Wechnia", but by the game itself as "**********", seemingly based on unused data for Tails. Starline's color scheme is taken directly from Wechnia, his name is a literal take on the name "**********", (or "star-line") the black streak in his fur resembles the red streaks all over Wechnia's fur, and the choice of a platypus seems to be inspired by how certain Wechnia sprites would make the bottom of his muzzle yellow and loosely resemble shading for the underside of a flat bill, alongside platypi being Australian like echidnas.
In the Japanese version of Star Fox Adventures, some of the dinosaur tribes have different localized names from their original names. For example, the CloudRunners are referred to as the クラウド族 (Kuraudo-zoku), which translates to "Cloud Tribe". The EarthWalkers are referred to as the アソーカ族 (Asōka-zoku), which strangely seems to translates to "Ahsoka Tribe". "Ahsoka" is a name of Sanskrit origin that means "pure and hopeful". 
Out of the 4 main survivors, Zoey was the only one to change voice actors, changing from Alésia Glidewell to Jen Taylor. Despite this, almost all of the pain and death sounds you hear in the game are actually lines leftover from Alésia and not Jen. It's unknown if this was intentional as she is not credited in Left 4 Dead.
Sonic the Hedgehog 4 was originally going to be a mobile exclusive game under the title "Sonic the Portable" before being made into a cross platform game marketed as a sequel to the classic trilogy on the Genesis. This name remains left in the game in Carnival Street Zone where it is displayed on the ferris wheel in the level background on a blurred out neon sign.
Inside the files of the US Deal or No Deal: Special Edition for DS is a crude 3D model of a cuboid of ice.
The 3DS version of Imagine Babyz has unused content pertaining to what seems to be an unreleased 2D shoot 'em up titled Sound Rider. There is a title screen, multiple ship and bullet sprites, and two MOD tracker-styled songs.
