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.
The game was originally designed for a 4 Mbit cartridge, only for it to get cut down to 2 Mbit due to a worldwide chip shortage at the time. As a result, over half of the game's content was cut, leading the developers to make the remaining three levels notoriously mazelike in order to artificially extend the game's length.
The resulting product was a disappointment to both Sega and Osomatsu-kun author Fujio Akatsuka. Hideki Sato, who designed the Mega Drive, stated that Hachamecha Gekijou reflected poorly on the system thanks to it being its first title that wasn't a port of an earlier arcade game, and that it made him wonder if he had poorly designed the system's hardware. Akatsuka, meanwhile, already treated video games with open scorn, but Hachamecha Gekijou outraged him so much that he threw an ashtray at an unnamed developer.
Dragon Rock, Star Fox Adventures' final SpellStone area, was considerably larger and more complex in earlier versions of the game than in the final release. Both the leaked December 2000 build of Dinosaur Planet and also the E3 2002 kiosk version of Star Fox Adventures show that Dragon Rock featured a slew of additional areas that didn't make the final cut, such as an underground quarry area where Sabre/Fox would have to save the imprisoned EarthWalker from being eaten by a group of creatures known as Skeetlas, and also a large underground mining facility Sabre/Fox could only access by flying the imprisoned CloudRunner to the top of the titular Dragon Rock (a giant tower in SFA) at the center of the map. In SFA's case, Fox would have to solve a puzzle that would involve powering the portal at the back end of the room that would grant him access to Drakor, Dragon Rock's boss.
In the final release, both the underground quarry and the mining facility were removed: Fox simply frees the EarthWalker from a corral on the surface by hitting a switch located nearby the imprisoned HighTop from across the map, and Fox immediately skips to fighting Drakor after being dropped off by the CloudRunner at the top of the center tower. Funny enough, when Fox lands on the platform in Drakor's arena, it bizarrely shows electricity spewing around him, which was a remnant of the portal he was supposed to activate in the cut mining facility segment in order to access the boss.
The SFA dev team admitted on a 2003 Rare Scribes that Dragon Rock was massively scaled back from its intended design due to "approaching deadlines", presumably referring to the impending Microsoft buyout in 2002. It's worth pointing out that the mining facility area can actually be found in the files of the final retail release of SFA and surprisingly in a much more complete state than the kiosk's version, which indicates that the dev team continued working on this area in the final months of development but ran out of time to polish it to their liking.
In the PS2 version of MaxPlay Classic Games Volume 1, you can find a full pirated copy of Metroid Fusion in the folder "DELETE_ME". It was likely used to test the compilation's emulator.
In the files of Jelly Belly: Ballistic Beans, there are a set of 4 images that, when pieced together, make a crude image of someone being hit with a Wii remote with the caption "DONT[sic] HIT PEOPLE".
The game features an unused sixth hidden tape called "Herd of Mouth with Wooly the Sheep" that can only be accessed by exploiting the game. However, the tape is entirely comprised of Wooly sitting at a table while occasionally making minor gestures for ten minutes, suggesting that the tape was added purely to mess with exploiters.
Sonic Mars was a concept game that was cancelled for unknown reasons (though it likely had something to do with the limitations and ultimate failure of the Sega 32X), before eventually being reworked into what eventually became Sonic X-treme.
In 2007, two different scripts were leaked for the cancelled game. The first, written by Michael Kosaka, focused on the 3D graphics and notably included elements from the 1993 Sonic the Hedgehog television series (referred to by fans as "Sonic SatAM"), which was airing at the time. The second, written by Don Goddard, focused more on how the game combined 3D graphics with 2D sprites, and dropped the SatAM elements.
Additional animations were considered for the T-Rex, including roaring and a kick inspired by Sonic the Hedgehog, but these were scrapped to keep the animation rigid.
The Google Chrome dinosaur game is coded to conclude after playing for 17 million years straight, the same amount of time between the dinosaurs' evolution and extinction. This is, quite obviously, not realistically possible for any player.
In the files of the Neopets promotional game Pepsi Surf Football, you can find a shortened audio clip from a trailer for the film Lilo & Stitch, featuring a narrator saying "His name is Stitch" with "Back in Black" by AC/DC playing in the background.
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.
On August 16th, 2023, Cruise Elroy revealed that he usually sketches out concepts for Annalynn before implementing them in game. While many of the sketches shown are present in the final release, some unused concepts are also shown, such as an orange character named Annabelle (presumably a second player character) and Mappy-style trampolines.
Among these sketches is also Mike drawn with an expression mirroring that seen in the "Today I Will" meme.
Icicles were planned to be a hazard in the Ice Grave area of the game, but they were cut as Cruise Elroy thought that the area's wind and ice physics were enough to make the area challenging. Sprites for the icicles were produced, however.
Unused power-ups include an hourglass to slow down the Venom Brigade and a potion which would shrink them and render them harmless for a short time. They were scrapped due to being too similar to the final game's pickaxe powerup, but sprites for them were produced.
When the Street Fighter skin set was released for Fortnite, a back bling of an arcade machine was released that would show attract mode footage from Street Fighter II. Coinciding with this, a texture was found in the data, under the name "FrenchFry" showing animation from Family Guy in a similar format. While many believed this confirmed that Peter Griffin was planned to be featured in the game alongside a back bling that would show Family Guy clips, it was later stated by a reputable Fortnite leaker named FNBRUnreleased that the Family Guy content could've only been added for one of two reasons: to annoy dataminers, or to internally test the Street Fighter graphics in a humorous way.
Griffin would eventually be added to Fortnite in Chapter 5. To coincide with his addition, a video titled "Peter Griffin Seeks Fitness Advice From Meowscles" was released: animated in the same style as "Family Guy", it features Griffin getting a fitness exam from Fortnite character Meowscles, ending with him drinking an expired bottle of Slurp Juice in order to become "swole", explaining his design in the game. The expiration date on the Slurp Juice is a reference to the date when the leak originally happened, and the trailer also used the leaked back bling. Despite appearing to be animated by Family Guy's animation department, the video noticeably does not feature any spoken dialogue, or voice acting by Family Guy creator and Griffin's voice actor Seth MacFarlane.
The Game Boy version of Yoshi has an unused Bullet Bill piece. It is not present in the NES version.
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. 
Dragon Rock, Star Fox Adventures' final SpellStone area, was considerably larger and more complex in earlier versions of the game than in the final release. Both the leaked December 2000 build of Dinosaur Planet and also the E3 2002 kiosk version of Star Fox Adventures show that Dragon Rock featured a slew of additional areas that didn't make the final cut, such as an underground quarry area where Sabre/Fox would have to save the imprisoned EarthWalker from being eaten by a group of creatures known as Skeetlas, and also a large underground mining facility Sabre/Fox could only access by flying the imprisoned CloudRunner to the top of the titular Dragon Rock (a giant tower in SFA) at the center of the map. In SFA's case, Fox would have to solve a puzzle that would involve powering the portal at the back end of the room that would grant him access to Drakor, Dragon Rock's boss. 
In the files of Jelly Belly: Ballistic Beans, there are a set of 4 images that, when pieced together, make a crude image of someone being hit with a Wii remote with the caption "DONT[sic] HIT PEOPLE".
In the files of the Neopets promotional game Pepsi Surf Football, you can find a shortened audio clip from a trailer for the film Lilo & Stitch, featuring a narrator saying "His name is Stitch" with "Back in Black" by AC/DC playing in the background.
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.
On August 16th, 2023, Cruise Elroy revealed that he usually sketches out concepts for Annalynn before implementing them in game. While many of the sketches shown are present in the final release, some unused concepts are also shown, such as an orange character named Annabelle (presumably a second player character) and Mappy-style trampolines.
Some cut bonus items include a candle, diamond, emerald, and drill. However, the candle was reused for the 2016's mode bonus items.
Icicles were planned to be a hazard in the Ice Grave area of the game, but they were cut as Cruise Elroy thought that the area's wind and ice physics were enough to make the area challenging. Sprites for the icicles were produced, however.
Unused power-ups include an hourglass to slow down the Venom Brigade and a potion which would shrink them and render them harmless for a short time. They were scrapped due to being too similar to the final game's pickaxe powerup, but sprites for them were produced.
When the Street Fighter skin set was released for Fortnite, a back bling of an arcade machine was released that would show attract mode footage from Street Fighter II. Coinciding with this, a texture was found in the data, under the name "FrenchFry" showing animation from Family Guy in a similar format. While many believed this confirmed that Peter Griffin was planned to be featured in the game alongside a back bling that would show Family Guy clips, it was later stated by a reputable Fortnite leaker named FNBRUnreleased that the Family Guy content could've only been added for one of two reasons: to annoy dataminers, or to internally test the Street Fighter graphics in a humorous way.