Jeremy Clarkson was originally going to narrate the Halo Warthog autovista experience in Forza Motorsport 4, but it was cut from the game in favor of having Cortana, from the Halo series, narrate it. However, the Jeremy Clarkson Warthog narration can be viewed online.
The international versions of Wangan Midnight Maximum Tune and its sequel have the Porsche Gemballa cars rendered inaccessible due to licensing issues. However, some players were able to hack Wangan Midnight Maximum Tune 2 machines to make those cars accessible. And, thanks to the ability to make backups of cards, and the "discarded card" system (in which a card that has had all 60 of its plays used up can be used to create two partially-tuned clones of it), Gemballa cards spread to WMMT 2 players around the world; all you needed to do to be able to use a Gemballa was to find a player who was willing to sell or give away a used-up Gemballa card to you. However, in the international version of WMMT 3, all data pertaining to Gemballa cars was removed, so attempting to transfer a Gemballa card to WMMT 3 would simply give you an error message and eject your card back out.
Gran Turismo 2 was planned to have a drag racing mode, but it was removed late in development, making 100% Completion impossible. Some of the drag cars were left in the game, though. The Eiger rally courses were also cut, although reinstated as Eiger Nordwand in GT5. GT3 also had the Lamborghini Diablo GT, which was rendered inaccessable (outside of cheats) in non-Japanese versions, due to licensing conflicts.
Driver has the dummmied-out city of Newcastle (where Reflections is based in real life), which the protagonist is shown driving through during the ending credits. In the PC version, it can be accessed by hacking.
The PS1 game Formula 1 97 had Jacques Villeneuve replaced by "Driverone Williams" due to licensing issues which existed at the time. However, Murray Walker's commentary referring to Villeneuve by name remains on the disc (and can sometimes be heard when the game crashes whilst a piece of commentary is playing). Lines referring to the ill-fated Mastercard Lola team and its drivers Vincenzo Sospriri and the hopeless Riccardo Rosset also remain. The arcade commentator also has many many lines that are never heard in the game.
Diddy Kong Racing has a number of surprising elements buried in its data, including a logo from the game's pre-release life as "RC Pro-Am 64." Hacking also revealed an unused level called Horseshoe Gulch, which doesn't seem to fit anywhere in the game (except possibly Dino Domain), and is obviously far from finished - all characters (including Wizpig) appear, and drive aimlessly; a stone arch is completely untextured; and progress in the level is prevented by a dead end.
The game has a dummied-out version of the Mario Circuit track from Double Dash!!, likely scrapped because of the limited amount of retro tracks that could be used without there being too many tracks (with the game having a limit of four tracks for each previous game). Its graphics are blank and untextured, so everything lacks the proper color, and bizarrely, even though the music that the track should have plays in DS's versions of the Luigi Circuit and Yoshi Circuit tracks from the same game, the title screen music plays if the track is accessed with a cheating device.
The game contains a few unused versions of Nitro tracks, such as a pinball level (presumably an early version of Waluigi Pinball), a version of DK Pass without snow (which replaces the rolling snowball obstacles with boulders), and a slightly different version of Wario Stadium (with most differences being related to obstacles).
The system files of Underground 2 contain the folder of the vehicle S2000. Unlike the other cars, it is missing several files needed to appear in game. Additionally, several areas that are blocked due to construction are fully functional but lead to dead ends or teleporting your vehicle back to the real track.
2005's Need for Speed: Most Wanted has a big list of traffic vehicles that never made it into the normal traffic roster. Despite that, they still appear normally in Drag races. Additionally, there's a folder of a stock BMW M3 with all files ready to be used. There's also a folder with the Nissan 350z, although, like the S2000 from Underground 2, it is lacking several files.
In the PlayStation version of Need For Speed III: Hot Pursuit, selecting a higher difficulty level would result in all police Corvettes being replaced by a police version of the Lamborghini Diablo SV. Said AI-controlled Pursuit Diablo still exists in the PC version of the game, but is unused. Furthermore, Hot Pursuit mode was completely cut out of the PlayStation version of the game.
The LEGO Racers track Knightmare-athon is notorious for being the sole track in the game to not have any shortcuts of any kind. However, hacks of the map data show floating item bricks where there were shortcuts planned.