References to other missing game modes and items litter the code, including multiplayer options to have destroyable doors and even walls (it's unclear how this was supposed to work), a missing mode called "Touch The Crate," and an extra singleplayer mission called "Retaking The Institute."
Golden Eye 1997 may be one of the single largest cases of this trope in action, as it had a boatload of content rendered inaccessible before the game came out:
The Byelomorye Dam mission had roughly half of its original objectives scrapped during development, as the designers believed it would have been too difficult for a first level. The most notable of these objectives was "Tower 1", a remote island with a mounted wall-gun from the later levels that you can still see a far distance away from the dam overpass. Originally, Bond would have had to take a boat out to the island, blow up a machine and possibly retrieve bungee jumping equipment needed to leap off the dam (it's also speculated that it may have been in the building with the commandant). The player would have had to get explosives to destroy the gun from the ultimately rather useless checkpoint with the satellite dish, halfway through the level. Bond also would have had to destroy the truck halfway through the dam mission; when it stops at a depot, you were supposed to plant explosives on it.
The pointless platforms with ladders in the Surface levels were originally supposed to be used by snipers. This was never implemented because the Goldeneye AI can't fire over railings, so they could only attack if the player actually climbed the ladder.
The game was originally intended (in the first few months of its development) to be an on-rails shooter, in the style of Virtua Cop, but this was changed. The Silo level also would have been an on-rails shooter where you would take out enemies from different platforms; Frigate seems to have been designed with this in mind too.
Bond would have fought Xenia Onatopp in the "Frigate" stage, but she was deleted from the final game. Presumably, she was the "helicopter pilot" character who only appears in multiplayer. Instead, you fight her in the "Jungle" stage. She is still mentioned in the "Frigate" mission briefing, though, and the area where you plant the tracker on the helicopter is placed suspiciously next to a large, open area that could have doubled as a boss room, and has an unused spawn location for the helicopter pilot character.
You could also access a ton of new weapons and items using a Gameshark or Action Replay. Evidently, these items were part of expanded subplots that were deleted, but the items themselves remained in the game data. These included the gas keyring from The Living Daylights, a watch communicator (to talk to Q?), micro cameras and much more. Several weapons were also found to be embedded in the data.
Other weapons were intended to appear but have no level assigned to them; the Shotgun originally appeared in Archives but is only accessible via the All Weapons cheat, the Magnum was available without killing Natalya on Jungle, and the Taser appeared several times.
The infamous "Citadel" multiplayer test level, which Rare denied existed for years. Hackers cracked the game's code and found an incomplete stage, then built the necessary setup files for the level to work normally.
The "All Bonds" cheat, which would have unlocked all the classic James Bond actors for multiplayer, but became Dummied Out when it became impossible to secure all the necessary licensing rights. The textures remain in the ROM, but the character models themselves were removed; Hackers have managed to create a partially-functional "All Bonds" mode by applying them to other models.
The "Line Mode" cheat is present and fully functional, but there's no longer any way to properly unlock it; there's a button-press cheat, but that doesn't permanently add it to the list. This would seem to indicate an entire missing level.
Tying in with the "missing level", it's long been rumored that there was an entire Casino level cut from the game (and replaced by the "Silo" flashback level). The evidence pointing to this includes the aforementioned missing cheat mode and the existence of several unused items in the game code, including a gold bar, casino chips and a camera. Fans have speculated that the level would have consisted of Bond planting cameras at the casino and meet with Xenia.
Dam, Cradle and Statue were also originally set up as 2-3 player multiplayer levels (complete with player starts and weapon spawn locations), but they can't be accessed without a cheat device. This is why Cradle has three walkways, while the real antenna only has one. The beta version also included a multiplayer version of Frigate called "destroyer," which helps to explain why the La Fayette in game appears to be an American Kidd-class destroyer. Cradle eventually became a playable multiplayer level in Goldeneye Source, a mod for Half-Life 2.
Ten old ZX Spectrum games from back when Rare was Ultimate Play The Game could be played in an emulator. This was apparently never intended to be on the actual release, but it was merely rendered inaccessible.
There were a number of enemies that were modeled, textured and coded before being cut in both Half-Life and Half-Life 2. One such monster, Mr. Friendly, was cut from the original Half-Life because his attacks would include pounding the ground to knock the player's weapon out of his hand, knocking his glasses off and raping the player to death.
Another monster cut from Half-Life 2 was the Hydra, which was cut because although it looked cool, it was no fun to fight since as the developer put it "You'd just see this blue blob doing something vague, then you're dead". There were also some coding issues, since it would impale whoever it attacked. A few of the areas it was meant to be fought in were reused for the beginning of the uprising in the retail game.
No screenshot on either of the original Half-Life or Half-Life 2 boxes actually exists in the games.
In c3a1 of Half-Life, noclipping at the elevator displays a fully functional elevator, but crashes the game as there's no map pointer for it.
The multiplayer map Crossfire, when accessed while playing story mode, has a security guard that can be interacted with. The reason for his placing remains unknown. The same map has a hidden room that is inaccessible via normal means, even in multiplayer.
In Halo, the Covenant Engineer was cut so late that the files were left on the discs. The canonical novels which make up Halos expanded universe include them though, and they are in Halo 3: ODST. The only reason they were cut was that the developers felt the tech couldn't convey them well enough for an emotional impact.
Same goes for the Flood Juggernaut in Halo 2. The files are still on the disc, so with some modding you can put them in the game and fight them.
There was a planned monster called the Drinol that didn't make it past the concept stage. It was supposed to be a Shout-Out to the Hulks from Marathon. The Flood Tank form in 3 is similar, though.
Halo 3 had the deleted "Guardian Forest" level, part of which was made into the MP map Guardian, another part of which was merged with The Covenant.
Previews of Halo 2 showed black Spec-Ops Elites (supposedly a higher rank) like the first game, but in the final game, all Speccies have indigo/violet armor.
The Halo 2 level Metropolis was instead going to feature an enormous tiered city referred to as Earth City. The city was changed to its final version in order to make it more realistic.
The flamethrower that was a "new feature" in the PC/Mac port of Halo was almost finished and is usable on the two levels Library and Keys in the Xbox version, with the use of a trainer with the spawn all weapons in game key. The weapon model is essentially the same as the one of Halo PC/Mac (just has a different ammo counter), it doesn't do any damage (except melee attacks with it) and there are no biped movement animations well holding it other then standing straight as if at attention with the weapon at your side pointed at the ground.
The three Marathon games have a few pieces of old data in them, including:
In Infinity a level had a double dummy, with a missing terminal and a hint that the text left in the resources was not the intended text for the terminal (a couple members of the dev team later gave us the real text)
The first game had a text string in a mission end terminal that, in the resources, was pure gibberish that included a character string (it was %r if you were wondering), that if the string is copied into another terminal, would display the same line (it's believed that this is a remnant of an attempt to make the first game have multiple endings or paths).
The level "Fatum Iustum Stultorum" has an inaccessible dummy room in the upper left corner, only visible in a map editor. Was this supposed to be the player's starting point?
The Hound, a planned companion to the Hunter, which was featured in the manual and some screenshots, and the Armageddon Beast (a Nigh InvulnerableJuggernaut type monster, not to be confused with the actual game Juggernaut), which was pictured in the Marathon Scrapbook.
The level "Come and Take Your Medicine" has an extra destroyable circuit that does nothing, a door that can only be opened by enemies, and a large meaningless outdoor area, indicating that more stuff was planned for this level.
Left 4 Dead apparently was going to have zombie dogs in the game, but was cut. Proof of this is in the sound files in the PC version; there are sound effects for the dog still left in the game.
The beta version of the game was going to have Zoey show some romantic interest to Francis, but it was cut out due to play testers deeming it as too distracting. The files for the conversation are still present (example: Zoey tells Francis she knows how to cut hair. Francis in the beta version of the game had long hair). However, one beta dialogue still plays out in the game, but it makes no sense in context; if Louis is shot by Francis, Louis may yell "Go on! Do it again, fat man!" The beta version of Francis showed him with a beer belly, while his final version does not have it.
Similarly in the sequel, there are unused lines by the survivors and some NPCs that indicate a part of the map the lines used was going to be more than what it appeared to be. One example is Whitaker having lines that suggest he was going to give the survivors covering fire as they got him some cola. However, he doesn't give any cover whatsoever.
A more well known example came from the chopper pilot in the No Mercy campaign. After saving the survivors, he would complain about feeling cold and then he would become sick and become a zombie. This was during a time in development when all the campaigns in Left 4 Dead were going to be connected, but since play testers felt frustrated that their rescue was a failure, Valve cut the lines, but they can still be heard in the game's files. However, the Crash Course DLC begins with the helicopter crashing because Zoey killed the infected pilot. Crash Course ends with them escaping in an armoured van and the 'next' campaign starts on a road blocked with abandoned cars.
The demo version of Left 4 Dead 2 also had every weapon and item that wasn't supposed to be in the demo dummied out but could be spawned via console commands. The Grenade Launcher is a more obvious example since it lacked textures and there was no official model for the fired grenade (which showed ERROR instead). A patch disabled the ability to spawn these items until the retail version of the game was released.
There were also a few maps cut out of the final game, such as Survival versions of a few crescendo events. Various mods re-add these maps.
The sequel also made it possible to splash other survivor players with Bile Bombs (summons a horde of zombies on the target or area) and there's actually unused lines with the survivors getting angry that the thrower smeared them in bile. However, this ability was cut out due to the feature making using bile bombs too frustrating. A console command in the PC version can be used to enable this feature if cheats are turned on.
Laser sights were going to be used in the first Left 4 Dead game but was oddly cut out in the final version (some modded servers re-enable this feature, but solely as a cosmetic addition). The sequel made the laser sights a complete feature.
Early on in the development for Left 4 Dead 2, survivors were able to find ammo packs, which they could deploy and the whole team could get ammo. This was cut out due to most people finding the idea useless. The console command to give the ammo pack still exists, but it does nothing. The texture for the ammo pack is also still present in the game's files, which was just a reskin of the health kit.
In The Parish campaign, the first two maps were going to be a large single map until Valve decided to split the area into two joined maps. The beta version of the single large map can be played by loading it from a console command. Aside from no sky textures and some model errors, the CEDA trailer by the bus station had a different design and would spray something on the survivors when they entered it. There was also a heavy machine gun near the switch to shut the alarm off, which is not present in the final version of this area.
The Sacrifice DLC contains unused audio for Bill which appears to be his final words as he slowly dies. The context of these lines appear to be for when the player has Bill sacrifice himself at the finale of the campaign and it goes quite well when it's compared to what happens to Bill in the comic version. It could have been possible for Bill to say these lines if he dies in the end, but they are never used and he just uses his standard pain screams in the ending.
Thanks to being rushed out the door to meet its release date, Jurassic Park: Trespasser had a host of dummied out features. A beta version of the game showed what was deleted from the final version:
For starters, the developers originally created a wide range of possible emotions for the dinosaurs in the game, so that they could react accordingly with what was going on in the world. In practice, though, this caused the dinosaurs to stay paralyzed in place because they couldn't decide between moods, so almost every dinosaur in the game was permanently set to maximum hostility as a quick fix.
The game was originally supposed to run at a much higher framerate than what it eventually ended up being. The sheer number of textures in the game (as well as conflicts with the game engine) forced the designers to severely scale back the framerate between the beta and final version.
Early on in development, it was proposed that the game would have a third-person perspective instead of a first-person one, and it's still possible to enable the third person view with cheats. In practice, however, this not only makes it even more difficult to aim your weapon and grab items, but it also reveals that the player character's model is just a floating pair of breasts and one arm - which is all you'd see in first person anyway.
The physics system in the game caused a bug with the game's melee weapons. Simply walking around while having a melee weapon unequipped would cause said weapon to knock into the player and deal damage (because it Sticks to the Back). The developers worked around this by removing mass from all the melee weapons, which also made them useless in combat - except for one, which is Nedry's mace.
The "Pine Valley" level, which was deleted from the final game (but, inexplicably, is the level you play through in the demo for the game). Pine Valley was supposed to take place between the worker town and docks levels (explaining why the terrain of the two levels is so different and the starting point of the latter looks nothing like the ending point of the former). It was going to feature Site B's geothermal powerplant, but the dinosaur AI didn't work properly in interior spaces (they kept getting stuck in walls) and the physics engine couldn't handle the mechanical puzzles the level was supposed to contain, so the level was unceremoniously cut.
There are files found in World at War that hint at both a campaign related to British forces and requisite British weapons, but these are nowhere to be seen in the final product.
Modern Warfare had several levels that were scrapped late in production, but can still be viewed by accessing the PC version's mod tools to view the remnants. Most of the scrapped content centered around the American campaign, with at least three more missions focusing on different protagonists fighting in Riyad'h (including a Cobra helicopter pilot and a Marine). The most notable dummied content, though, was a segment of the opening training mission that involved Soap running through an obstacle course and further practicing his weapon proficiency, all while being verbally berated by two drill instructors. There's even a mod that allows you to play the cut content!
Sgt. Newcastle: (before Soap blows up a broken-down car with C4) It seems my ex-wife was kind enough to donate her car to further your education, Soap. Mac: (while running the obstacle course) You crawl like old people screw! I've seen Sandhurst commandos move faster than you lot!
Also cut out was a series of very strange lines from Captain MacMillan, presumably meant to be spoken during the Pripyat missions. They're hilarious, but it's hard to imagine under what circumstances he would have said them.
Cut audio clips (which are still in the game files) for "Mile High Club" indicate that the VIP the team rescues, Kriegler, is the same man who was shipping the nukes on the cargo ship in "Crew Expendable". The mission was also moved to a post-credits extra because Infinity Ward couldn't fit it into the main story (that, and it's likely that it was just too difficult for what was intended to be the second level of the game).
Dummied out mission objectives for "Aftermath" indicate that the player was originally supposed to locate their commanding officer and find "secure overhead cover". This was changed for the final version of the level, where Pvt. Jackson always dies.
Modern Warfare 2 also had several levels and parts of levels cut. One of these involved you following Capt. Price into the sub at the end of "Contingency", and was presumably cut to make the part where Price launches the nuke more surprising. There was also a mission where you rode a motorcycle (which would probably be similar to the snowmobile part), and one where you fought Russians on the ISS (which IW cut for being too out-there, making it just a cutscene where the ISS is destroyed by Price's nuke).
It's worth noting that Call of Duty: Black Ops included a motorcycle section at the end of the mission "Vorkuta" that plays similarly to the snowmobile section in Modern Warfare 2, suggesting that they reused the data.
Likewise, in Modern Warfare 3, the second mission of the game has the player board a Russian sub to disable it, using the exact same level layout as the submarine Price boards in 2, indicating the level section was reused.
Call of Duty: Ghosts opens with a space station being attacked by an enemy force and a pair of astronauts forced to defend the station. It's believed that this utilized the scrapped content from Modern Warfare 2's original ISS level.
How much of that content was planned to be used and how much of it was there as a Red Herring for people who look specifically through files for dummied out content, will only be known to the TF2 developers. However, the "medic follow" lines were put into use since the Engineer Update (July 8, 2010).
One feature was the "Melee Dare" lines where if you took out your melee weapon with another player nearby, they'd say a line daring you to a melee duel. This was cut for an unknown reason, but some servers enable it as a mod. The lines were later scripted in (among other lines) when a player aims at an enemy with a melee weapon and chooses the "Battle Cry" speech.
This was later implemented back in with some classes through the duel minigame; upon challenging another player, one of several responses (a few of which are melee dare sounds) will play.
Source Filmmaker was on the original Team Fortress 2 demo—since its discovery, amateur filmmakers everywhere have used it to make videos. A version of Source Filmmaker was officially put into beta with the Pyromania Update.
When the community found out how to read the file names to indicate what would come next before any official announcements, Valve started to place strings which seemed to change the entire course of the game (namely, strings for Raidbosses and RPG elements). Initially thought to be Valve's way of covering up their subsequent updates by placing false herrings, it turns out that these were in fact unused features. The Raidboss strings would be used when the Horseless Headless Horsemann was introduced (and every other subsequent boss thereafter) and finally, the RPG elements would be used in the gamechanging Mann Vs Machine Update (where players can purchase upgrades to level up their weapons). The latter ended up spanning a good few years, with (in hindsight) sporadic hints dropped left and right.
Jedi Knight II: Jedi Outcast's multiplayer option allows for a player to use RGB colour customization on a small array of player skins in the Player Customize menu where one can change their name, lightsaber, etc. However, this RGB option can be applied to some player skins that aren't in the Customize menu, which can quite easily be done by opening the Command Window. Doing this results in much fascination amongst newbs, and is mostly done by highly interesting and dominant figures of authority and internet heirachy who like to feel unique amongst thousands and thousands of Kyle Katarns.
Jedi Academy also completely missed some player models from the Player Customize menu, although the models themselves are completely playable. The models include the RGB compatible non-customizable skins, as well as the Jawa model, which, because of the problematic AI, has developed a cult following where a Jawa will bop up and down into and out of a crouch. This has been called the "Jawa Dance."
One of the multiplayer levels on Taspir has towers with elevators that constantly go up and down. One of these towers has an elevator that doesn't go to the final floor, as it was never finished for some reason.
Also note that, similar to Half-Life above, it is usually impossible to replicate official screenshots of the game without cheating - they typically show the player character using two lightsabers or a double-bladed lightsaber in levels that come before you have that option. This may be a result of double dummying-out; the code exists in the game (and can be activated through the console) for Jaden to pick up the lightsabers of defeated Reborn, which means you could have potentially been able to dual wield as early as the tutorial level (of course, unless you used the console further that second one would invariably be red).
Coding exists for "realistic sabers" in both Outcast and Academy, wherein any humanoid enemy (and you yourself) will lose body parts and die just from slight contact with the blade of someone else's lightsaber. A console command exists to activate the setting, but in Academy it was made harder to activate after a patch.
Unreal features a number of creatures that were never used in the game or are incomplete, a non-functioning weapon's file and a level named "Gateway" representing a huge empty space station with no item, no monster and non-functioning stargate-like teleporters made of water. It has a number of cool features and can be accessed through the level editor.
Unreal Tournament, having all of its spiritual predecessor's data (sans maps and music) even has several items, (such as a silencer and the nali fruits and seeds to restore health) models, textures, codes and sounds which weren't used in the final game nor in the console ports or the official free bonus packs. Being a moddable game, there are several maps which incorporate such things.
Unreal Tournament 2004 has the "Vehicle CTF" mode, which is, obviously, Capture the Flag with vehicles. There are no official maps of this gametype, however, so it isn't playable in the actual Tournament mode and requires a downloaded custom map to be listed in the Instant Action menu.
In Doom II, the source code contains commented-out code that would have allowed for the doors in the two secret levels to function like they did in Wolfenstein 3D.
Quake II has a fully-working Power Screen, which worked the same as the Power Shield, but only in one direction. It wasn't included in any level, outside of third-party ones.
Quake III: Arena has a Grappling Hook, similar to that of the past CTF incarnations, almost fully working. The Expansion Pack Team Arena even had a portal device. Had it been completed and polished, (the item was modelled, but the portals weren't visible in-game) it would have predated Narbacular Drop and Portal.
There are unused sound files for announcing a win by a character named X-Ray.
Quake is an interesting example: the very first release was the Beta version, QTest, which was a three-map multiplayer only version of the game with slightly different game mechanics from the finished product. Not long after QTest was released on the Internet, however, enterprising hackers discovered a way to access/spawn enemies from the game code inside the levels; giving players a sneak peak towards what would be available in the then upcoming single player shareware release. These early dummied out versions of the enemies include an ogre which uses a nailgun instead of a grenade launcher, and a massive version of the leaping demon. The latter was in fact so big that it actually caused the QTest to crash when it attacked you, which is presumably the reason why the ones in the finished Quake are so much (comparatively) smaller.
Duke Nukem 3D has dozens of textures and sprites in the data files which are not used anywhere in the main game. User-made levels, however, are more than happy to make use of them.
The manual for Turok 2 describes a monster called a Hunter, but you don't see it anywhere in the game. It might have been a prototype of the Leaper enemy seen in Level 4.
Strife had a number of unused resources buried in the .wad files, such as graphics for a gas grenade and some bits of voice acting.
The original Medal of Honor had several missions/levels scrapped due to time and/or processing constraints, including a radar train (possibly during or after the Railgun Greta mission), a jet aircraft facility (although the game still shows the jet fighter film clips), and Colditz Castle. The last one was resurrected as the Panzerknacker Unleashed bonus mission in Underground, and its music was used in Allied Assault.
Allied Assault also has an unused level that can be accessed through the console; it may have been intentionally left in as a Secret Level.
PAYDAY: The Heist has several unused audio and text files showing that there would have been other heists that took place on a boat or other areas. Word of God says they had tinkered with several heist ideas during development before they settled on the final version of the game.
The sequel has a safehouse that acts as a tutorial level. There's several walls in the basement that look suspicious due to the wall being brick instead of cement. People have discovered through no clip that there's actual rooms behind the walls; most of the rooms are empty but one of them has a few cameras inside and a camera icon sprayed onto the floor, which may suggest that the player would have been able to practice skills related to cameras.