History GoodBadBugs / FirstPersonShooter

* Another MookMaker enemy introduced in Doom 2 is the Pain Elemental, which can't hurt the player directly; it floats around spitting out Lost Souls, flaming skull enemies from the original game, and releases three when it dies. However, if there are too many Lost Souls already active (about 20), the Pain Elemental won't be able to spawn any more Lost Souls; it just floats around doing its spitting animation but not achieving anything. In some cases players will exploit this by intentionally killing as few Lost Souls as possible along the way to a tricky area with many Pain Elementals, rendering the area much easier by making it impossible for them to spawn more. This is also useful in 100% kill runs in the console versions[[note]]PC versions as of ''Doom'' v1.666 don't count Lost Souls towards the kill percentage, but the console versions are based on the code for the UsefulNotes/AtariJaguar port, which still does as it predates the release of ''Doom II'') to reduce the number of 'extra' Lost Souls the player has to hunt down and kill.

* Valve's Achievements have their fair share of bugs, but one in particular stands out — the Identity Theft Achievement is a Spy Achievement that is given for backstabbing the enemy you're disguised as. Stabbing someone with the Your Eternal Reward awards you this Achievement instantly.

** There's also the [[http://doomwiki.org/wiki/Barrel_suicide barrel suicide glitch]] in earlier versions of ''Doom''. To wit, a monster that is injured (but not killed) by an exploding barrel will retaliate against whatever caused the barrel to explode, in the first place. If it was a fellow creature, it'll attack that monster. If it ''itself'' destroyed the barrel that injured it, it'll attack ''itself'', either going berserk with their projectile attacks (with the zombie monsters) or "tearing themselves apart" with their melee attacks (with most other demons). While fixed in latter versions of the game, it brings in a new facet to the classic infighting strategies inherent to the game.
** In the final level of DOOM II, [=MAP30=]: Icon Of Sin, the final boss spawns enemies to do his dirty work. If the map is played on the ''Nightmare!'' difficulty or with the respawning enemies-flag, the monsters will respawn ''outside'' the level harmlessly. This can make the difficulty of the final level slightly easier than if the enemies actually were resurrected where they could hurt you again.
** The grand-daddy of good bad bugs in Doom is [[DiagonalSpeedBoost strafe-running]], which every Doom player ever has done since about two weeks after the game came out: due to a fairly simplistic physics engine, instead of running straight in the direction you want to go, it's faster (nearly root 2 times faster) to face at an angle away from the direction you want to go, and strafe towards your destination while running forward - for example, turn 40 degrees to the right, then run forward and strafe to the left.

** The original Doom engine is intended to make it impossible for the player to pass through any gap up to 32 units in size, but in fact, any gap between two walls which is exactly 32 units in size can be glitched through with trial and error. For a long time this was thought to only be possible where one edge of the gap is made up by a corner, but in fact the trick can even be pulled off when there is no wall to 'lean' against, it's just harder. This trick is used in many speedruns, though (as with many such tricks) it sometimes reduces interesting runs to boring-yet-frustrating competitions of which runner can trigger a glitch faster.
** Similarly, there are several levels in both Doom and Doom 2 designed with the expectation that a player will not be able to grab an item on a low and quite small raised platform without finding a way to get on top of the platform, but in fact it is possible to grab the item by running into the platform at a precise alignment, or rubbing alongside it. The classic example of this is the platform in the penultimate room of Doom 2's first level, which contains a powerful weapon in multiplayer mode; the intended method of accessing this weapon is to activate an elevator platform behind it, stand on it and wait for it to rise, and then jump onto the weapon, but in fact, all moderately skilled players simply 'bump' the platform to get the weapon.

*** The same idea also allows monsters to unlock key doors that are only locked from one side - the monster bumps into the door, which allows its collision box to go through the door, causing it to open when they move away. The faster a monster is, the thicker doors they can open by this method; it's extremely rare outside of custom maps, however, as only a few doors in the original games are locked from just one side, and in almost all of these cases the locked side is within spitting distance of its key.
** The aforementioned archvile enemy can also attack the player directly; any time the player is in the archvile's line of sight it can cause a magic flame to start burning around the player. A short time later, if the player doesn't manage to get out of the archvile's line of sight, the flame 'explodes' and the player takes significant damage...and is also thrown into the air. This mechanic can be exploited to perform an 'archvile jump' to cross a gap or reach an area which would usually be inaccessible, another trick used in several speedruns.
** Another MookMaker enemy introduced in Doom 2 is the Pain Elemental, which can't hurt the player directly; it floats around spitting out Lost Souls, flaming skull enemies from the original game, and releases three when it dies. However, if there are too many Lost Souls already active (about 20), the Pain Elemental won't be able to spawn any more Lost Souls; it just floats around doing its spitting animation but not achieving anything. In some cases players will exploit this by intentionally killing as few Lost Souls as possible along the way to a tricky area with many Pain Elementals, rendering the area much easier by making it impossible for them to spawn more. This is also useful in 100% kill runs to reduce the number of 'extra' Lost Souls the player has to hunt down and kill.
** ''Doom'''s engine handles {{hitscan}} weapons in such a way that the first shot is always perfectly accurate with subsequent shots deviating, resetting after a short time with no shots fired. Skilled players can use this quirk to snipe enemies with the chaingun, a weapon that is usually horribly inaccurate at long ranges.
** The way the engine handles usable switches or doors is also ripe for exploitation. [[http://doomwiki.org/wiki/Linedefs_can_be_activated_regardless_of_player%27s_Z_position The game does not care whether the player is at the same level as the object]], so long as they can reach the wall it's on they can activate it. One level in ''VideoGame/{{Hacx}}'' in particular can be completed in one second because the exit switch is, while initially hidden, right next to where the player spawns - all they need to do is turn around and hit the use key at the air behind them, and they'll trigger it.
** The ''VideoGame/BrutalDoom'' mod has a particularly hilarious one. The invisible tracer spawned by the Offend command (used for "hit" detection when attempting to piss off demons) can trigger linedefs... which means it's possible to tell certain impact-activated doors and switches to go fuck themselves, ''[[SugarWiki/FunnyMoments and they will open]]''.
*** It gets better than that -- for all intents and purposes, the tracer is treated the exact same as firing a bullet. One that's [[CherryTapping slow to fire and deals next to no damage]], but it's still damage. This can lead to stopping a charging Lost Soul in its tracks by swearing at it, or even [[https://www.youtube.com/watch?v=Q_yv8j7JmTw killing enemies with low-enough health by flipping them off]].
* ''Doom 64'' runs on an engine that is still similar to, but noticeably distinct from, that of the original games. One of its new additions is scripting, allowing for all sorts of cool things like views through security cameras and the player's view shifting from first-person to third-person. One of the downsides to this, however, is that only one script can run at a time - until that script finishes playing out, any future scripts will simply ignore being triggered. In particular, this allows you to beat the BrutalBonusLevel, "Hectic", without fighting a single enemy. Normally, the red and blue keys trigger scripts that begin ambushes (lowering walls to reveal arachnotrons and then lowering the ceiling over almost all of the room one time once they're dead for the red key, draining the damaging liquid in the room and lowering far walls to reveal hell knights for the blue key), but there are also several pickups in the starting room that trigger inescapable deathtraps if you go for them. The green and blue armor are straight to the point, respectively just crushing you or blowing you up, but the Soulsphere takes its time trapping you in place and very slowly lowering a sequence of crushers, from the far edge of the room all the way to right on top of you, to let you know just how badly you fucked up - but if you abuse collision detection to just barely edge yourself into grabbing the Soulsphere without getting trapped when the floor around it lowers, you can take advantage of that long script to rush to grab the keys while their scripts are being preempted.

** The ''VideoGame/BrutalDoom'' mod has a particularly hilarious one. The invisible tracer spawned by the Offend command (used for "hit" detection when attempting to piss off demons) can trigger linedefs... which means it's possible to tell certain impact-activated doors and switches to go fuck themselves, ''[[CrowningMomentOfFunny and they will open]]''.

** When pseudodogs charge towards you and attack, there is a very small chance that the physics engine will glitch and you will be [[CrowningMomentOfFunny propelled several hundred meters straight up at high speed]]. This has never been fixed despite having been present since the first game was released.

* ''[[VideoGame/FirstEncounterAssaultRecon F.E.A.R.]]'' is coded so that picking up a new weapon gives a lot more ammunition than picking up a second copy of a weapon you already have as a refill (generally two or three full mags for a completely new weapon, versus at most one full mag for an untended copy of a weapon and half or less of one for a copy dropped by an enemy). However, that amount if determined when you pick up the weapon and remains the same even if you drop it and pick it up again. A sneaky player can, with a nearly empty SMG and three more on the ground, drop his currently equipped SMG, then pick up one of those three [=SMGs=] to force the game to consider it a "new weapon" with more ammo in it. Then pick the first SMG back up to combine their ammo and repeat the process for the other [=SMGs=], until you've picked up every gun. This results in a lot more ammunition for the player than was intended by the developer. The glitch still works in ''F.E.A.R. 2'', though is much harder to take advantage of between the fact that the ability to just drop weapons without picking up a new one was removed, and fewer separate weapon pickups overall with the addition of actual separate ammo pickups.

** If you get Gaige's Anarchy up high enough (>550, only possible with the Slayer of Terramorphous) and use a weapon that's low-accuracy to begin with (less than 30), her accuracy will apparently make a RealityBreakingParadox, break the game, and enter the territory of EpicFail. The targeting reticule (which should, at this point, be the entire screen) will shrink to a pinpoint, but instead of being incredibly precise the bullets will follow no logic whatsoever. Bullets fly perpendicular, land behind you, or even zigzag in midair. Anything you hit is bound to be an accident, and also bound to ''die instantly''. [[note]] 600 Anarchy stacks gives you a ''1,050 %'' damage increase. [[/note]]

** The Jupiter factory complex in ''Call of Pripyat'' was originally supposed to be a major Monolith base. However, a spawning bug made all the Monolith troops disappear, leaving only a few pesudodogs and hidden anomalies in the (fully modeled and explorable) buildings. This also has the effect of [[NothingIsScarier making the whole area incredibly creepy and memorable]] [[ParanoiaFuel due to it being dark and deserted]]. Some mods, most notably S.M.R.T.E.R, fix this bug.

* Mann vs Machine had a glitch where, if you buy an ubercharge canteen, go outside of spawn, unequip it, go back to spawn, use your canteen, then purchase another ubercharge, the ubercharge becomes permanent until round failure! This can also be done with crits.
** Before an update, Mann vs Machine only had 8 of the 9 classes given a robotic counterpart (the Engineer was left out) - but sometimes the game would spawn robot Engineers anyway, simply giving them the BLU team model for the regular human Engie.
** And now, with the new Crusader's Crossbow, Fully upgrading it results in a every single syringe loaded being fired out in a Spread Shot minus the spread.

* Ladies and gentlemen: [[https://www.youtube.com/watch?v=c1WpcxgCsCQ The Breadbomb]]. In case you can't watch the video, it seems that, in Upward, any kind of prop aside from player-related ones will trigger the Payload blast. This includes the various kinds of bread that spawn when someone teleports. So, someone had the idea to place the teleporter exit right above the pit, and have the bread fall in, which would ''always'' result in a gigantic explosion. This causes the final point to be capped for BLU and turns the cart invisible. The cart can still be pushed, and the game continues normally until the cart reaches the second-to-last point, where BLU wins, or if RED successfully defends (if they're not too dumbstruck by the explosion.) Sadly, this glitch no longer works.

** It's known in the ''[[Franchise/TheLegendOfZelda Zelda]]'' community as the Crooked Cartridge trick, where it enabled massive sequence breaking in ''[[VideoGame/TheLegendOfZeldaOcarinaOfTime Ocarina of Time]]'' by preventing hit collision detection on removable barriers. Its direct sequel ''[[VideoGame/TheLegendOfZeldaMajorasMask Majora's Mask]]'' also had a Crooked Cartridge effect on game creation or copying, where it produced the "Second Hero" and "Third Hero" glitched new games where Link obtained items he otherwise wouldn't have at that point of the game and where he died if he was attacked or healed due to having no hearts at all. The later could be somewhat fixed by collecting a full heart container, giving the player one measly heart instead of the default three.

* In ''VideoGame/{{BioShock|1}}'', attacking Sander Cohen's masterpiece will cause the lights to dim and music to play before he attacks you. This can happen any time after you complete his masterpiece, including after you (optionally) kill him in his apartment. Thus, this artist will ''come back from the dead'' to defend his work.

** An infinite ammo glitch, which allowed players to fire unlimited rounds without reloading. Doubly ridiculous because it applied to killstreaks, meaning there were '''AC-130's''' flying around with '''infinite, rapid-fire artillery-sized guns''' mounted on the side that were balanced entirely by their need to reload after a certain number of shots. Potentially the MoreDakka champion of video gaming.

*** In general, ''Halo 2'' was less a game with glitches and more a glitch with a lot of gameplay. Many of the maps were surprisingly easy to escape, sometimes with easter eggs located in these areas. Online combat was (and still is with ''Anniversary'') made of glitches; [=BxR=] (which allowed one to melee and fire almost instantly, meaning an almost instant kill from no damage) being the most infamous, and more time was likely spent in the maps looking for exploits than killing each other. It even spawned one of the prototypes for gaming creepypasta: The Ghosts of Halo, of which there's still no firm explanation, just a lot of confusion. The general theory is that they're the half-deleted prototypes for matchmaking bots, which still have yet to appear in the franchise. This is supported by ''VideoGame/{{Halo 3}}'' also containing them on maps known to have had bots used for playtesting in-house. However, as the only way to get them to spawn is to create lag so massive it screws things up, it's extremely hard to really study them.

** It was also possible in one level to beat a level in three seconds. When you spawned, you could see the map's goal (The Eye) twinkling off in the distance. Down two speed potions, run like hell towards it, and jump with a mighty jump. Then walk out the exit, located about twelve steps away. Since the level also had those damn zombies you couldn't hide from (or kill without holy water or fire arrows) this was a hell of a lot more fun.

* ''VideoGame/JediKnightJediAcademy'' has the same issue. It's entirely possible to kill about half of the game's bosses by standing where you start the battle, using Sense, and firing an automatic weapon at their face until they die.

* ''[[VideoGame/FirstEncounterAssaultRecon F.E.A.R.]]'' is coded so that picking up a new weapon gives a lot more ammunition than picking up a second copy of a weapon you already have as a refill (two or three magazines, compared to just one or even less than half of one). However, that amount if determined when you pick up the weapon and remains the same even if you drop it and pick it up again. A sneaky player can, with a nearly empty SMG and three more on the ground, drop his currently equipped SMG, then pick up one of those three [=SMGs=] to force the game to consider it a "new weapon" with more ammo in it. Then pick the first SMG back up to combine their ammo and repeat the process for the other [=SMGs=], until you've picked up every gun. This results in a lot more ammunition for the player than was intended by the developer.
** The glitch still works in ''FEAR 2'', but is much harder to pull off because the command to just drop a weapon was removed, and on top of that it's rare to find weapons themselves on the ground in more than one instance rather than separate ammo pickups.

** The previous level ends with you knocking over some beer mugs to create a distraction. However, the animation for doing so is incredibly simple, tossing the mug in an arc upwards and forwards so it lands on a table below - but nothing is done to prevent you from hitting the button again before the animation finishes, at which point it will reset and have the mug move in that arc again, starting from wherever it was in the last arc. Mash the button and the mug will be constantly sent higher and higher into the sky until you eventually can't see it. And then it will break in mid-air anyway as soon as it falls as far as it was supposed to from where it was when you last pressed the button, the people below you still reacting as though they could see and hear it happening.

* The concept of [[RocketJump Rocket Jumping]] started off as a glitch in ''VideoGame/{{Quake}}'' that didn't get fixed before launch, but which players quickly learned to exploit to get around the map more quickly and to get to places that would otherwise be unreachable. Nowadays, [[AscendedGlitch most FPSes implement some variant of rocket jumping]], some even have items that can be equipped to reduce the player damage taken in the process.

*** The lesser known "Campfires" meme is based on a spawning bug in the unpatched ''Shadow of Chernobyl'' where many NPCs would accidentally spawn inside bonfires in some of the settlements. Sometimes ''entire villages of [=NPCs=]'' could be found dead around campfires.

*** In general, ''Halo 2'' was less a game with glitches and more a glitch with a lot of gameplay. Many of the maps were surprisingly easy to escape, sometimes with easter eggs located in these areas. Online combat was (and still is with ''Anniversary'') made of glitches; [=BxR=] (which allowed one to melee and fire almost instantly, meaning an almost instant kill from no damage) being the most infamous, which contributed massively to the {{Hatedom}} for the Battle Rifle, and more time was likely spent in the maps looking for exploits than killing each other. It even spawned one of the prototypes for gaming creepypasta: The Ghosts of Halo, of which there's still no firm explanation, just a lot of confusion. The general theory is that they're the half-deleted prototypes for matchmaking bots, which still have yet to appear in the franchise. This is supported by ''VideoGame/{{Halo 3}}'' also containing them on maps known to have had bots used for playtesting in-house. However, as the only way to get them to spawn is to create lag so massive it screws things up, it's extremely hard to really study them.

* ''[[{{VideoGame/Tribes}} Starsiege: Tribes]]'' contained a physics engine bug which allowed players to "ski" downhill at high velocities via well-timed use of the jump key. This exploit quickly became a gameplay staple, achieving such popularity that, after initially fixing it, the bug was very quickly [[AscendedGlitch deliberately integrated it]] into the sequels, incorporating it into the tutorials and modifying the games' physics engines to make skiing easier.

* ''[[{{VideoGame/Tribes}} Starsiege: Tribes]]'' contained a physics engine bug which allowed players to "ski" downhill at high velocities via well-timed use of the jump key. This exploit quickly became a gameplay staple, achieving such popularity that the game's developers, rather than fixing it, [[AscendedGlitch deliberately integrated it]] into the sequels, incorporating it into the tutorials and modifying the games' physics engines to make skiing easier.

** ''Tribes: Ascend'' has its own, which lets you [[https://www.youtube.com/watch?v=NmpYJXfCKCI wear an inventory station as a hat]]

** It's known in the Zelda community as the Crooked Cartridge trick, where it enabled massive sequence breaking by preventing hit collision detection on removable barriers. Its direct sequel Majora's Mask also had a Crooked Cartridge effect on game creation or copying, where it produced the "Second Hero" and "Third Hero" glitched new games where Link obtained items he otherwise wouldn't have at that point of the game and where he died if he was attacked or healed due to having no hearts at all. The later could be somewhat fixed by collecting a full heart container, giving the player one measly heart instead of the default three.

** An infinite ammo glitch, which allowed players to fire unlimited rounds without reloading. Doubly ridiculous because it applied to killstreaks, meaning there were '''AC-130's''' flying around with '''infinite, rapid-fire artillery-sized guns''' mounted on the side. Potentially the MoreDakka champion of video gaming.

** A bug existed that allowed players to easily get the medal/achievement for shooting down enemy air support in multiplayer. Waiting until a helicopter was crashing, having already been 'destroyed', then unloading on it with a LMG, each shot counted as you 'destroying' the killstreak. With decent aim and good timing, you could max out this challenge in seconds.

* In the manor house level in ''VideoGame/MedalOfHonor: Frontline'', you can sometimes get past the entry guard (typically by sneaking to a particular angle and shooting him before he spots you) without blowing your disguise, allowing a StealthRun of the level, or at least most of it.

** The previous level ends with you knocking over some beer mugs to create a distraction. It's possible to mash the action button as much as you want, constantly sending the mug higher and higher into the sky until you eventually can't see it. And then it will break in mid-air anyway as soon as it falls as far as it was supposed to from where you last pressed the button, the people below you still reacting as though they could see and hear it happening.

** Because of how the navigation meshes (determines how the AI can move around the map) are made, sometimes they aren't perfect. Depending on where you stand, you can cause the infected to suddenly become oblivious to you, even if you are only a few feet away from them. This is because AI zombies are programmed to chase and attack on you sight but they will suddenly just stop and flail about if they can't physically reach you. This can happen on the most mundane objects such as a table.

*** When Louis deals the final blow to a Tank, he may say "That was for Bill!" even if Bill is alive. This line is supposed to be used in the finale of The Passing campaign, since chronologically at that point, Bill is dead.
* ''VideoGame/KillingFloor'' has a glitch with weapons that can change their fire rate. Firing repeatedly in semi-auto mode and then switching to full-auto can trigger a glitch where the gun will have [[http://www.youtube.com/watch?v=GC5iLNRy4Gw no recoil]]. Newly-introduced weapons with selectable fire rates benefit less from this, as they still go through their recoil animations even when firing under this glitch's effects. The glitch is also impossible to perform in singleplayer.

* ''VideoGame/DukeNukem3D'': Duke gets squished when the height of a room is under a specific value in pixels. Duke also dies if he gets outside the map. Combining these two rules in two overlapping rooms could allow Duke to safely teleport above or underneath a room where he could have died otherwise.
* The concept of [[RocketJump Rocket Jumping]] started off as a glitch in Quake that didn't get fixed before launch, but which players quickly learned to exploit to get around the map more quickly and to get to places that would otherwise be unreachable. Nowadays, [[AscendedGlitch most FPSes implement some variant of rocket jumping]], some even have items that can be equipped to reduce the player damage taken in the process.

*** The lesser known "Campfires" meme is based on a spawning bug in the unpatched ''Shadow of Chernobyl'' where many NPCs would accidentally spawn inside bonfires in some of the settlements. Sometimes ''entire villages of NPCs'' could be found dead around campfires.

** During the fight with [[spoiler:Namir]], when he goes to leap over a wall, pressing the "takedown" button right when he's at the top of the wall leads to [[http://www.youtube.com/watch?v=4v1X5F0d58I&feature=related Jensen knocking him out with a single punch]]. It only works in the original release, though; the Director's Cut may have fixed it but the bug still exists. [[https://www.youtube.com/watch?v=4DgPZtwH1WI Observe]].
** Occasionally, a character may bug out and collapse in on itself, folding into a compact upper torso sitting there on the ground, as demonstrated in [[http://www.youtube.com/watch?v=CbRYVWgF9L4 this]] ''Podcast/FourPlayerPodcast'' video.

** ...to the point, naturally, of being ''intentionally'' invoked in certain third-party levels, such as level 24, "The Haunting" from ''Icarus: Alien Vanguard'', or level 23, "Hatred" from ''Requiem''. Of course, in enhanced versions of the engine, this bug is fixed, although some allow you to enable it again.

** During the fight with [[spoiler:Namir]], when he goes to leap over a wall, pressing the "takedown" button right when he's at the top of the wall leads to [[http://www.youtube.com/watch?v=4v1X5F0d58I&feature=related Jensen knocking him out with a single punch]], even on the hardest difficulty. Only works in the original release, though; the Director's Cut fixes it.

** ''Modern Warfare'' actually had a bug so big it affected ''other games''. In August 2010, some players discovered that by booting up [=MW2=] and then using the XBox Quick Launch bar, they could sidestep the copy-protection of several SNK games available in the Xbox Live Arcade. This allowed them to play the full versions having only downloaded the demos.

** In one quest in ''Call Of Pripyat'', you need to help Vano take out some Monolith in a bookstore. As he stands outside the store, he is likely to be attacked by wild animals, so to prevent the quest from failing before you even start, the developers made him invulnerable and had him ignore enemies while he waits for you. [[NightmareFuel The result]], [[http://www.youtube.com/watch?v=imoMRuVYMFM well...]]

** The [[MemeticMutation "You see Ivan" meme]] is based on a very funny glitch where [=NPCs=] use the wrong animation for holding a weapon, most famously holding their left hand in front of the barrel of a pistol as if it's a rifle.
*** Likewise, both the "GET OUT OF HERE STALKER" and "I SAID COME IN, DON'T STAND THERE" memes were created due to a humorous dialogue looping bug where the characters in question would [[WelcomeToCorneria repeat the lines over and over again]].

** There's another one early in the game at Chomper's Dental in the Medical Pavllion. There's a door bell the you press to open the door (for those who've bit their controller in half in frustration looking for the key needed to do this, it's in the office itself - use telekinesis through the broken window), at which point a splicer spawns in the next room in order to ambush you. I repeat: ''when you press the doorbell, it spawns a splicer''. Happy crashing, folks.

* For a few days after the Jungle Inferno update was released, eating the Dalokohs Bar or Fishcake as Heavy with the Gloves of Running Urgently (which was changed to slowly drain the Heavy's health at a rate of -10 health per second) equipped, then rapidly switching between whatever minigun the player has equipped and the GRU would cause the Heavy's health to overheal beyond the normal limit (the process starts off very slow but as the overheal rises higher and higher it speeds up[[note]]The exploit could also be done with the Eviction Notice, which was also changed to slowly deplete health but a -6 health per second, meaning the process of accumulating overheal would take much longer.[[/note]]). Using this exploit, it was possible to overheal to '''millions''' of health, making the Heavy almost completely [[MadeOfIron unkillable]] as a result, with the only thing being able to kill him in one hit with such an astronomical amount of health being a backstab from a Spy. It was quickly patched a few days after it was discovered.

[[Folder:Other]]

[[folder:''Deus Ex'']]

[[folder:''Doom'']]

[[folder:''Team Fortress 2'']]

** Occasionally, a character may bug out and collapse in on itself, folding into a compact upper torso sitting there on the ground, as demonstrated in [[http://www.youtube.com/watch?v=CbRYVWgF9L4 this]] FourPlayerPodcast video.

** Also from ''Call of Pripyat'', a character named Crab is scripted to stay laying in a bed on the Skadovsk until you complete a particular side quest midway into the game. Sometimes when you load a game where you saved on the Skadovsk, Crab can be found out of bed. [[http://www.youtube.com/watch?v=FkpFE19-4CA He then starts living up to his name]]. It's as hysterical as it is [[NightmareFuel terrifying]].

** In the version of the Source Engine that games in the Orange Box run on, Valve added a feature to the game that was intended to nerf bunny hopping by applying force to slow you down whenever you're moving too fast. However, it's possible to trick the game into applying the force in the wrong direction. And due to the fact that you're now moving even faster than before, the next time you hop game applies a stronger force. Which lead to the creation of [[https://www.youtube.com/watch?v=pBql39nXmno&t=37s Accelerated Back Hopping]], which allows players to get more speed than they could ever hope to get with bunny hopping alone. [[NiceJobBreakingItHero Nice job fixing that, Valve.]]

* A very memorable glitch in ''VideoGame/{{Doom}} 2'' has the Arch-Viles, a MookMaker who can resurrect dead enemies, creating "ghost monsters" if they try to revive monsters killed by being crushed (like with a DescendingCeiling). Ghost monsters can clip through some walls and are completely invulnerable to all but explosion splash damage (from either a rocket or an {{exploding barrel|s}}). It's a rare (and sometimes frustrating) occurrence that somehow manages to appeal to avid ''Doom'' enthusiasts...
** ...to the point, naturally, of being ''intentionally'' invoked in certain third-party levels, such as level 24, "The Haunting" from ''Icarus: Alien Vanguard'', or level 23, "Hatred" from ''Requiem''. Of course, in enhanced versions of the engine, this bug is fixed, although some allow you to enable it again.
** There's also the [[http://doomwiki.org/wiki/Barrel_suicide barrel suicide glitch]] in earlier versions of ''Doom''. To wit, a monster that is injured (but not killed) by an exploding barrel will retaliate against whatever caused the barrel to explode, in the first place. If it was a fellow creature, it'll attack that monster. If it ''itself'' destroyed the barrel that injured it, it'll attack ''itself'', either going berserk with their projectile attacks (with the zombie monsters) or "tearing themselves apart" with their melee attacks (with most other demons). While fixed in latter versions of the game, it brings in a new facet to the classic infighting strategies inherent to the game.
** In the final level of DOOM II, [=MAP30=]: Icon Of Sin, the final boss spawns enemies to do his dirty work. If the map is played on the ''Nightmare!'' difficulty or with the respawning enemies-flag, the monsters will respawn ''outside'' the level harmlessly. This can make the difficulty of the final level slightly easier than if the enemies actually were resurrected where they could hurt you again.
** The grand-daddy of good bad bugs in Doom is [[DiagonalSpeedBoost strafe-running]], which every Doom player ever has done since about two weeks after the game came out: due to a fairly simplistic physics engine, instead of running straight in the direction you want to go, it's faster (nearly root 2 times faster) to face at an angle away from the direction you want to go, and strafe towards your destination while running forward - for example, turn 40 degrees to the right, then run forward and strafe to the left.
** Wall-running involves strafe-running along a wall that's aligned exactly north-south or east-west on the game map; it doesn't trigger instantly and so is useless on short walls, but usually after a bit of time you will suddenly start moving way faster than is usually possible. In some maps, this can conveniently make long jumps possible.
** The original Doom engine is intended to make it impossible for the player to pass through any gap up to 32 units in size, but in fact, any gap between two walls which is exactly 32 units in size can be glitched through with trial and error. For a long time this was thought to only be possible where one edge of the gap is made up by a corner, but in fact the trick can even be pulled off when there is no wall to 'lean' against, it's just harder. This trick is used in many speedruns, though (as with many such tricks) it sometimes reduces interesting runs to boring-yet-frustrating competitions of which runner can trigger a glitch faster.
** Similarly, there are several levels in both Doom and Doom 2 designed with the expectation that a player will not be able to grab an item on a low and quite small raised platform without finding a way to get on top of the platform, but in fact it is possible to grab the item by running into the platform at a precise alignment, or rubbing alongside it. The classic example of this is the platform in the penultimate room of Doom 2's first level, which contains a powerful weapon in multiplayer mode; the intended method of accessing this weapon is to activate an elevator platform behind it, stand on it and wait for it to rise, and then jump onto the weapon, but in fact, all moderately skilled players simply 'bump' the platform to get the weapon.
** Again similarly, items placed close to walls can sometimes be grabbed from the other side of the wall with a precise bump or rub. This is exploited in speedruns on several levels to grab keys.
*** The same idea also allows monsters to unlock key doors that are only locked from one side - the monster bumps into the door, which allows its collision box to go through the door, causing it to open when they move away. The faster a monster is, the thicker doors they can open by this method; it's extremely rare outside of custom maps, however, as only a few doors in the original games are locked from just one side, and in almost all of these cases the locked side is within spitting distance of its key.
** The aforementioned archvile enemy can also attack the player directly; any time the player is in the archvile's line of sight it can cause a magic flame to start burning around the player. A short time later, if the player doesn't manage to get out of the archvile's line of sight, the flame 'explodes' and the player takes significant damage...and is also thrown into the air. This mechanic can be exploited to perform an 'archvile jump' to cross a gap or reach an area which would usually be inaccessible, another trick used in several speedruns.
** Another MookMaker enemy introduced in Doom 2 is the Pain Elemental, which can't hurt the player directly; it floats around spitting out Lost Souls, flaming skull enemies from the original game, and releases three when it dies. However, if there are too many Lost Souls already active (about 20), the Pain Elemental won't be able to spawn any more Lost Souls; it just floats around doing its spitting animation but not achieving anything. In some cases players will exploit this by intentionally killing as few Lost Souls as possible along the way to a tricky area with many Pain Elementals, rendering the area much easier by making it impossible for them to spawn more. This is also useful in 100% kill runs to reduce the number of 'extra' Lost Souls the player has to hunt down and kill.
** ''Doom'''s engine handles {{hitscan}} weapons in such a way that the first shot is always perfectly accurate with subsequent shots deviating, resetting after a short time with no shots fired. Skilled players can use this quirk to snipe enemies with the chaingun, a weapon that is usually horribly inaccurate at long ranges.
** The way the engine handles usable switches or doors is also ripe for exploitation. [[http://doomwiki.org/wiki/Linedefs_can_be_activated_regardless_of_player%27s_Z_position The game does not care whether the player is at the same level as the object]], so long as they can reach the wall it's on they can activate it. One level in ''VideoGame/{{Hacx}}'' in particular can be completed in one second because the exit switch is, while initially hidden, right next to where the player spawns - all they need to do is turn around and hit the use key at the air behind them, and they'll trigger it.
** The ''VideoGame/BrutalDoom'' mod has a particularly hilarious one. The invisible tracer spawned by the Offend command (used for "hit" detection when attempting to piss off demons) can trigger linedefs... which means it's possible to tell certain impact-activated doors and switches to go fuck themselves, ''[[CrowningMomentOfFunny and they will open]]''.
*** It gets better than that -- for all intents and purposes, the tracer is treated the exact same as firing a bullet. One that's [[CherryTapping slow to fire and deals next to no damage]], but it's still damage. This can lead to stopping a charging Lost Soul in its tracks by swearing at it, or even [[http://www.youtube.com/watch?v=Q_yv8j7JmTw killing enemies with low-enough health by flipping them off]].

* The original ''VideoGame/DeusEx'' did this on several occasions by utilizing unintended functions of the game engine, much to the joy of players:
** A great source of hilarity was screwing with the (all in-game) cutscene conversations through various means. You could throw a gas grenade ahead of yourself, initiate a conversation with an important NPC and watch as both JC and the other person mosh their heads up and down due to gas damage affecting (but not killing) them. You could sometimes get enemies to chase you into rooms with NPC's, begin a conversation and watch them wait patiently in the background before unloading their weapons into you. You could hack a security system, set off the alarm, watch as enemies come to check up on you through the cameras and hack a turret to mow them down without retaliation. You could also tranquilize an NPC, begin a conversation and watch as their dialogue is punctuated with sharp cries of pain.
** It's possible to skip the entire Liberty Island mission by picking up a gas grenade by the entrance to the island's pier, throwing it at the UNATCO base door (which can't be unlocked or blown up at this point) and watching as the soldier at the front desk runs outside to investigate (thereby unlocking the door and allowing you to waltz right in). It's a main component of speedruns.
** At the end of the Vandenberg base mission, speaking to Gary Savage results in a reward: an item and some Tech Points. If your inventory was full, JC says that he can't take the item yet and breaks out of the conversation to allow you to drop something. When you talk to him again, he tries to give you your quest reward: an item and some Tech Points. This can be exploited for infinite Tech Points by talking to him repeatedly and never actually dropping an item.
*** There is also a keypad in the Vandenberg base, one of the ones that you have to use to power up the robots online. Punching in the code will get you 200 skill points - every single time you use it. For even faster skill points, just hack the keypad with a multitool, and you can gain 200 skill points every time you click the mouse button.
** Saving bioelectric energy with the Enhanced Strength Aug. Whenever you're holding a heavy object that you can only lift while using the aug, go up to any wall and deactivate the aug. The aug will deactivate, but since you don't have enough room to drop the object, you will still be holding it. Thus, you can hold any heavy object this way without actually spending any B.E.
** Walking on mines. Go to a wall. Crouch, and attach a LAM. Face towards it, and step up onto the LAM. Attach another one to the side slightly, and step onto it. Reach down and remove the first. With care and patience, you can climb any wall. Much fun, especially in Hong Kong.
** Another glitch affected how multitools and lockpicks work. If you start to pick a lock/hack a keypad, and go to any menu that pauses the game, the lockpick will keep working at the same rate until you go out of said menu, even past the when it's supposed to stop. Using this, with some patience, you can open any door and disable any device you want, even with minimal skill levels.
** If you go to the inventory screen, then drag one of the items off the grid and close the screen without letting go, the next item you pick up will go underneath the original. This makes it possible to neatly sidestep the "specialize in only a few weapons" aspect of the gameplay, and carry around every weapon in the game.
** A bug in the Game Of The Year edition turns off the music on the second visit to New York. While the intended music is pretty badass, considering what is going on at the time of the game, silence works just as well.
** With a bit of preparation, [[http://www.it-he.org/deus3.htm you can make everyone in UNATCO try to murder the invincible vacuum cleaner]] to the point where they they eventually run out of ammo and try using knives, and while they're doing that, you can make Simons attack you until he runs out of ammo as well, lure him into the broom cupboard, and lock him in.
** In the downtown Paris core, it's possible to force Renault and his contact (the two men sitting in the hostel) to get up and sit somewhere else by destroying the chairs they're sitting on. Doing this enough will cause them to break into nearby apartments just to sit down, allowing you to pilfer everything inside without having to waste lockpicks.
* In ''VideoGame/DeusExHumanRevolution'', the combat system allows you to walk up to unsuspecting enemies and either kill them or knock them out with a scripted takedown animation (and later, you can learn a double takedown). It appears that the game scripters (whether intentionally or not) applied the same takedown animations to ''civilians''. This means that you can walk up to random people and perform hilarious moves, like [[http://www.youtube.com/watch?v=pEWoVsF0MIo&feature=related coldcocking a prostitute in the face while her friends look on]], punching someone so hard that it apparently breaks the sound barrier and [[http://www.youtube.com/watch?v=zYpuzHyEY8Q&feature=related taking down two civilians who've suddenly gained combat skills]].
** During the fight with [[spoiler:Namir]], when he goes to leap over a wall, pressing the "takedown" button right when he's at the top of the wall leads to [[http://www.youtube.com/watch?v=4v1X5F0d58I&feature=related Jensen knocking him out with a single punch]], even on the hardest difficulty. Only works in the original release, though; the Director's Cut fixes it.
** Occasionally, a character may bug out and collapse in on itself, folding into a compact upper torso sitting there on the ground, as demonstrated in [[http://www.youtube.com/watch?v=CbRYVWgF9L4 this]] FourPlayerPodcast video.