Deliberate example: There is no Leisure Suit Larry 4, largely because lead designer Al Lowe couldn't figure out how to logically continue the series from the third game on and chose to skip straight to the fifth one. Its nonexistence is a major plot point in the fifth game: at the end of the third, Larry finally found the woman of his dreams; at the start of the fifth, they're separated and neither of them actually knows why, leading to Patti trying to find the missing Larry 4 so she can figure out what the heck happened.
Another explanation is that Larry 4 was originally going to be a massively multiplayer online adventure game (in the early nineties!), but development never got off the ground as modem technology was still much too primitive at the time, so in the end the small minigames that were used to beta-test the project's online capabilities were packaged together and sold as The Sierra Network.
Leisure Suit Larry 4 turns up as a gag in several other Sierra games, but is actually a plot point in Space Quest 4. Vohaul smuggled his consciousness onto a disk of Leisure Suit Larry 4, and the Xenonian scientists are so eager to play it that they load it into the planet-controlling supercomputer. When you're in Vohaul's lair later in the game, one of the programs on his computer is "LSL4."
Same source says that it suddenly occurred to him at that time that skipping 4 would be a great marketing gimmick. He was right.
Star Fox 2, was never released, even though the Japanese version was practically completed. The plot would have continued the story from the previous game, and would have introduced Star Wolf as major antagonists. The game was canceled most likely due to the pending release of the Nintendo 64. Shortly after the game's termination, Star Fox 64 began development, and rebooted the storyline from scratch. Eventually the ROM was leaked to the internet, and an English Fan Translation was released.
Many computer games, after their initial publishing run, suffer from a problem somewhat unique to the medium. As Science Marches On, it can be quite rewarding to produce a Video Game Remake or Updated Re-release especially designed for newer computers, except that the source code and other assets of many commercial games are rarely held onto. For example, when the xu4 and Exult projects wanted to make source ports of Ultima IV and VII, Origin admitted that it had lost everything. And when Fallout Tactics was under development just a few years after the previous Fallout game had been released, it turned out that virtually all of the original game's 3D assets had been lost, and nearly all of it ended up being remodeled.
Speaking of Fallout, there's the original sequel to Fallout 2 made by Black Isle, code-named "Van Buren", which was about 85% done when Black Isle's parent company Interplay went bankrupt and the game was never seen again. Fallout 3 was eventually made five years afterwards by Bethesda, yet had nothing to do with Van Buren.
Fallout: New Vegas, however, does reference several concepts that would have been part of Van Buren, specifically Caesar's Legion, the Van Grafs and Hoover Dam. It's no coincidence that the development team for New Vegas was comprised of many former members of Black Isle.
Ultima X: Odyssey: It was canceled just four months after EA shut down Origin.
Many games that don't make it overseas are this to the foreign fans of a series who are cursed with hearing people who actually did get it talk about it, but will never play it themselves. If they're lucky, it's an old game that will get a Fan Translation. If not, they're screwed.
Some may eventually be released, even if in a different form, such as Final Fantasy II (part of Final Fantasy Origins for the Playstation) and III for the Nintendo DS and Super Mario Bros 2 (released in the US about 7 years later for the Super NES on the Super Mario All-Stars cartridge as "The Lost Levels").
The excessively violent and knowingly offensiveThrill Kill was pulled from distribution before it could offend, likely to avoid fears of a moral outcry, and because AO-rated games are not allowed on consoles. The game developers were rightfully annoyed by this; leaking a beta version of the game before releasing another fighting game, Wu-Tang: Shaolin Style using the same game mechanics, albeit marginally toned-down (i.e., LESS bloody and gory).
Several Amiga magazines were sent review copies of Putty Squad, and even the hardliners at Amiga Power graded it 91%—but the Amiga port never publicly surfaced beyond a coverdisk demo. Only an SNES version was published.
The Amiga version was finally released twenty years after initially planned, and for free - you can download it from System 3 on their promotional page
The fourth game in Atari's Swordquest series, Airworld, was never developed, probably due to The Great Video Game Crash of 1983. One of Parker Brothers' Return of the Jedi games also never made it past the concept art stage (the other unreleased game, Ewok Adventure, was discovered as a prototype).
Similar to the Leisure Suit Larry example above, there was an installment of Sam & Max: Freelance Police made called Sam & Max: Freelance Police!!. However, LucasArts cancelled it and it wouldn't be until another two years before Telltale Games would make a Sam & Max game. Like LSL above, the game is referenced in the Telltale Games series as a "particularly gruesome case".
The gruesomeness (and bitterness) around the LucasArts sequel is that the game was finished and already rated by the ESRB, before being caught up in the studio's decision to leave the adventure game business entirely.
The reason why many Sega Saturn classics like Panzer Dragoon Saga and Shining Force III have never been re-released is because Sega lost the original programming code for the games. Same for their System-16 (and then some) arcade games.
The original Sonic the Hedgehog for the Sega Genesis was first presented to the public as a playable tech demo at the 1990 Tokyo Toy Show. Yuji Naka said that the demo was planned to be included in Sonic Mega Collection, but were unable to because the original ROM for the tech demo was lost internally at Sega.
The infamous would-be Killer App for the Sega Saturn, Sonic X-treme, never made it to shelves – a result of the game's Troubled Production, which was riddled to the core with Executive Meddling – taking a near-fatal toll on the remaining programmer's health, due to which the game's development was discontinued shortly afterwards. Some assets – such as character sprites, level assets, and a early prototype of the "Project Condor" boss engine – have been leaked onto the internet, whereas other aspects (such as the original engine used for the game) have not seen release.
Yet another "lost video game": Despite what many fans believe, the Tiny Toon Adventures video game Defenders of the Universe never got past development stage. Dialogue and graphics were already produced at that time but Treasure simply terminated the project because publisher Conspiracy continuously held off the release date from 2001 all the way up to the spring of 2002, had it been finished. According to rumors, Conspiracy had major financial troubles that led them to pushing the release date numerous times, as well as terminating many of their other planned products. The company told Treasure that "business complications" was the reason they had to wait. It was not until eight years later when an image of a prototype version was shown, and still it looked incomplete (even though the ESRB already reviewed the game at the time of termination, possibly citing that the game was almost complete). Another project, Scary Dreams, was released the same year, but only in Europe (North America did not acknowledge that game's existence until three years later).
This happens constantly with embedded online games (mostly Flash nowadays, but some older ones used Shockwave).
Anything that was on Bonus.com in the late '90s/early 2000s, since the site is now defunct and most of the media on it (mostly games but some stories too) were endemic to it.
The BBC's website had a game that revolved around a allosaurus called "The Big Al game." It was very good for a web game. You started as a baby and had the option of staying near your mother, who moved on her own but would protect you if you were on the same tile, or venturing off. You moved one tile at a time searching for food and avoiding predators, including bigger allosaurus, and even your own mother if spent too long away from her. The game revolved around eating to get bigger. To eat, you find prey, then click to attack it and if the its strength bar is lower than yours you eat them and if it is higher you get injured. You could find a pile of hidden eggs for lots of free food, but they attracted other small predators, which is actually a good thing if your big enough to eat them (more easy food). There is also a carcass, but you died if you got to close because of rushing water or something. The second to last level involved searching for a sauropod migration ground and forming a pack with other allosaurus to take one down, and if you succeeded, you spent the last level mating as many times as you could to add to your score. To mate, you had to select the correct series of actions to woo the female, get it wrong and they attack, and if you selected a certain series of actions the female would ask you a random trivia question about allosaurus as a hidden joke, mate with you if you got it right, and attack you if you didn't. It's unknown why this game was removed, but now it's gone with few knowing it ever existed. Now you are one of them.
There was also another BBC game called "The Evolution Game" which would have been fun if it wasn't so brokenly hard. It was similar to "The Big Al game" in principle. You moved one tile at a time to find food or avoid predators, and in theory evolved over time into a human. In practice, it was impossible to make it past the first 3-4 life stages without staving to death or getting killed by a predator, which is probably why it eventually got taken down.
Millsberry was an advertisement game for General Mills. It ran for 6 years but shut down in 2010.
The Super Mario WorldromhackNotte Luminosa was taken down from SMW Central after it was revealed that the creator lied about having terminal cancer, in an attempt to get other people to make Let's Plays of the game.
The Eyewitness series of educational video games has one of these, if it ever was made. There is an Eyewitness Virtual Reality Shark game referenced to in the others, but no physical copies exist. Not even torrents. It's likely that this one game never was made.
There's actually quite a lot of Super Mario Worldromhacks that fall into this. Some examples include anything from before the time SMW Central was 'hacked' and wiped clean six or so years ago, any resources use in the Japanese hacks Ore World 2 and The Mario (since the authors websites have vanished along with their asm and code), anything from the first VIP 5 uploader (taken down because people used it for things other than VIP 5 submissions), anything from the original Japanese hack hosting/submission site (which vanished without a trace a few years back) and "Super Mario World Freedom" since the author's website has been taken down.
Many user-uploaded cities and components such as buildings and maps of past SimCity games like 2000 and 3000 (the first with an Internet component) have been lost as fan sites compiling those city files for others to play have shut down, bought by larger media companies that didn't care about those archives, or corrupted during site upgrades.
This happens to most MMOs once they shut down, unless fans are able to preserve them on fan-run private servers separate from the ones the creators maintained.
The Sims Online has been lost, but there have been attempts to restore it. The original attempt failed due to EA sending a cease-and-desist letter to the developers, but there are others trying with different engines.
There was a Metal Gear Solid port for the very unpopular Tiger Electronics Game.com console that was never released due to Konami being unhappy with the market performance of the platform (although some of its resources, code and level design found its way into Metal Gear: Ghost Babel).
Original plans for the game was for it to be called Metal Gear Solid III, to reinforce the game's censorship/editing of sequences theme (where did the missing II go?). This was soon abandoned, but early (internal) trailers still refer to it by this name.
There is a large and expensive chunk of that was modelled, animated, acted and complete, but cut from the game at the last minute as the 9/11 terror attacks happened towards the end of development (the scenes depicted Arsenal Gear laying waste to various New York landmarks). The model of the ruined New York and several of the props and character models used in the sequence can be looked at on the Documents of MGS2 disc, which also presents the script of the sequence; and the novelisation and comic book adaptations (released in the 2010s) retain this sequence. One cinematic from the sequence (a news report showing the Statue of Liberty's new resting place) was Dummied Out and was found by a mod group spelunking the PC version, but it is missing audio.
Metal Gear Solid Mobile, an award-winning Interquel set between Metal Gear Solid 1 and 2 with a whole new plot and game mechanics, was released just before touch-screen phones became the new thing, and thus was immediately rendered incompatible with everything. Since it was only available for digital download from companies that have now shut down and since phones of that era are impossible to emulate for various reasons, obtaining and playing it is almost impossible, even with piracy - only the NGage version is available on illegal channels and it requires a cracked NGage to play (a discontinued and highly unpopular piece of hardware that goes for hundreds even second hand). There are a few hacker groups working on it, but it's not likely to happen. It doesn't help that it was only available in Japan and to people using Verizon phones in America - so many people in Europe thoroughly missed out.
Piece Walker, a rather enjoyable promotional game for Metal Gear Solid: Peace Walker where you compete with another player to complete jigsaws, is down from the website and can no longer be played. It has not been released again in any form.
Similar to the Panzer Dragoon Saga example above, the Game Boy Advance version of Mega Man Anniversary Collection was cancelled because Capcom lost the original code for the original Game Boy Mega Man games and it proved too expensive to rebuild them.
The Atari 2600 game Saboteur was never released (would have been released circa 1980), but it was a selectable game in the Atari Flashback 2, released in 2005.
The MMORPG Star Trek Online has removed a number of missions from its game as Cryptic went back and started reworking them. The more notable one was "State of Q", where the player went back in time to the Battle of Wolf 359 to save Benjamin Sisko from the Borg of 2409, sent back in time by, presumably, the Iconians. Cryptic has stated, though, that they'd like to revisit this mission one day.
This trope applies to all versions of Freedoom, a project to make a game with 100% free content using the Doom engine, prior to 0.6.4. Before its release, a Doomworld community member discovered that one of composer Sam Woodman's contributions was plagiarized from the 1997 Duke Nukem 3Dtotal conversionThe Gate. 0.6.4 was soon uploaded with all of his music removed due to potential copyright infringement; and because his music was present in Freedoom for so long, all previous releases were taken offline.
Back in the mid 90s, Sega had the digital distribution service known as Sega Channel. Players would receive games straight to their Genesis via a TV signal. Unfortunately, no means of storing them was provided, so the games would be lost after the system was shut down. As as result, any Sega Channel-exclusive game versions were never dumped and are now lost forever. Among them is the Sega Channel-exclusive game Garfield: The Lost Levels and the American version of Mega Man: The Wily Wars.
By virtue of it being a satellite broadcast system, most of the games offered on Nintendo's Super FamicomSatellaview service haven't and will most likely never be re-released again. The service relied on broadcasting games and other pieces of media to those who ponied up the equivalent of an extra $140 or so to buy the peripheral in the first place. These games included unique versions of The Legend of Zelda: A Link to the Past (with their own unique plots), a Chrono TriggerGaiden Game styled after one of the minigames from the original title, a pseudo-sequel to F-Zero with new tracks and circuits, a variant of Excitebike with Mario as a playable character and much more. Due to the way the games were distributed, they could only be played if the person had a compatible memory pak and downloaded the title, meaning that once the service ended, very few copies of the games were left in existence. While some of the games (like the Zelda titles) have been preserved as ROM files and distributed online, plenty more have been lost in the ether.