Although Mario & Wario was released only in Japan, the game is entirely in English; a US release was planned, complete with box art, yet never materialized.
Of note is that the scrapped English box art features a logo identical to the one seen on the title screen, and that the game itself contains unused graphics for a Japanese version of the title screen with a logo identical to that seen on the Japanese box art. Combining this with the fact that Nintendo games' title logos rarely deviate from the box art (and vice-versa), this implies that the final game is actually based on an English prototype which was shipped to Japanese retailers by mistake.
EA had originally planned to add Gollum as a playable character. This ultimately did not come to fruition for the PC version and almost every console version of the game, but he was added to the GBA version of the game.
One leftover detail indicating his planned playability is an in-game results screen template with Gollum's face faded in the background, which is located in the Cheat Codes menu of the game. Every playable character in the game has a screen like this, with Gollum being the only non-playable character to have one.
In Sports mode, the Arnold football has an indicator above it to inform players that it must be grabbed and not hit. This graphic, however, is a crude placeholder graphic that appears to have slipped into the final game.
The layout of the Traffic Jam stage in Nickelodeon All-Star Brawl, consisting of a bus driving along moving asphalt that players can fall onto and either ram into the bus or get conveyed off-screen, while cars appear to the left and right to stand on, is recycled from a stage from Ludosity's first platform fighter Slap City called Public Transit, with the main difference aside from the Hey Arnold! theming being the absence of an equivalent to the Apathetic Frog helicopter.
The background music for the Slap City stage is also called "Traffic Jam" on the game's song selector (though the official Bandcamp release and the internal file names call the song "Noisy City" instead) and a side-scrolling stage reusing assets from Public Transit in Slap City's story mode is called "Traffic Jam".
In the files for the Neopets promotional game Alvin and the Chipmunks: How Many Chipmunks Could You Fit in Your Room Without Your Mom Finding Out?, you can find leftovers from the previous promotional game The Chronicles of Narnia: Prince Caspian Personality Quiz.
Crazy Cheebo: Puzzle Party was originally intended as a game starring the cult ringtone mascot The Annoying Thing/Crazy Frog titled Crazy Frog Collectables: Faces, following the naming scheme of Crazy Frog Collectables: Art School. However, the bankruptcy of publisher Mercury Games resulted in the game being shelved, with developer Cypronia stepping in to publish the games themselves with an original Crazy Frog-inspired character in place of the license. A leftover menu icon for the Crazy Frog revision of the game can be found in the files.
The SNES versions of Beethoven: The Ultimate Canine Caper! and Tom & Jerry both contain a dummy text file from the SNES version of The Hunt for Red October.
IN NOVEMBER OF 1984, SHORTLY BEFORE GORBACHEV CAME TO POWER, A TYPHOON- CLASS SOVIET SUB SURFACED JUST SOUTH OF THE GRAND BANKS. IT THEN SANK IN DEEP WATER, APPARENTLY SUFFERING A RADIATION PROBLEM. UNCONFIRMED REPORTS INDICATED SOME OF THE CREW WERE RESCUED. BUT ACCORDING TO REPEATED STATEMENTS BY BOTH SOVIET AND AMERICAN GOVERNMENTS, NOTHING OF WHAT YOU ARE ABOUT TO SEE... ...EVER HAPPENED.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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."
Although Mario & Wario was released only in Japan, the game is entirely in English; a US release was planned, complete with box art, yet never materialized.
EA had originally planned to add Gollum as a playable character. This ultimately did not come to fruition for the PC version and almost every console version of the game, but he was added to the GBA version of the game.
In Sports mode, the Arnold football has an indicator above it to inform players that it must be grabbed and not hit. This graphic, however, is a crude placeholder graphic that appears to have slipped into the final game.
The layout of the Traffic Jam stage in Nickelodeon All-Star Brawl, consisting of a bus driving along moving asphalt that players can fall onto and either ram into the bus or get conveyed off-screen, while cars appear to the left and right to stand on, is recycled from a stage from Ludosity's first platform fighter Slap City called Public Transit, with the main difference aside from the Hey Arnold! theming being the absence of an equivalent to the Apathetic Frog helicopter. 
In the files for the Neopets promotional game Alvin and the Chipmunks: How Many Chipmunks Could You Fit in Your Room Without Your Mom Finding Out?, you can find leftovers from the previous promotional game The Chronicles of Narnia: Prince Caspian Personality Quiz.
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.
Inside the files of the US Deal or No Deal: Special Edition for DS is a crude 3D model of a cuboid of ice.
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.
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.
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". 
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.
