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, deliberately integrated it into the sequels, incorporating it into the tutorials and modifying the games' physics engines to make skiing easier.
Tribes 2 also had a pretty hilarious bug that could get entire teams hooked on trying to trigger it. Not sure if it was ever patched, but it was possible to launch a Mobile Point Base clear off into the horizon. All you had to do was park it and let it latch itself into the ground, hurl concussion grenades at it, have somebody start to drive it and unhook it from the ground, and then it would suddenly fly away suddenly spinning on all 3 axes.
The class menu is also bugged, occasionally showing one class with the weapon and pose of another class.
Similarly, GunZ: the Duel contains bugs which may be exploited to prematurely terminate an attack animation, allowing certain attacks to be rapidly chained. This exploit has blossomed into a whole new form of gameplay, called "Korean Style" or "K-Style", which differs radically from ordinary GunZ gameplay.
For Halo 2 fans, K-style can be compared to using BxR or double shooting a battle rifle, except it requires combos of up to a dozen keys.
There are still some obvious animation glitches, but most players beg the developers NOT to fix them as they are hilarious to watch and usually harmless. The most famous of them is the Spycrab. Which is now referenced in a special, 10% chance taunt with the Spy's Disguise Kit. Thankfully, Valve opted not to fix it and spycrabbing is still possible.
Not to mention the quickly patched teleporter exploit, which enabled players to walk under the level and build sentry guns right below the enemy spawn points, essentially preventing enemy players from ever leaving their spawn rooms.
One amusing glitch occurred when the developers forgot to flag a server stress-test command for admins only. If used by an enraged player, it quickly buried everybody in the server under a massive pile of orange road cones until the server crashed.
One of the updates for the game shipped with an entirely new game mode that had a rather hilarious glitch: after a match goes to overtime, the Announcer immediately shouts "Overtime!" whenever the central control point's status changes. This glitch was so popular that even after it was fixed, Valve allowed servers to keep this behavior as an option.
There was also the (sadly patched after so long) "Crazy Legs" animation glitch, where the Scout's legs would, upon crouch double-jumping, freak out like (as Valve repeatedly describes it) two strips of raw bacon in a wind tunnel.
Another glitch that didn't last nearly long enough was one which allowed spies to use the console to build dispensers. It crept back in, along with Engineers being able to bypass building restrictions with the console. Then it crept back out again.
There are a few glitches with the spawn gates, although none of them are severe enough to be abused. Among other things, a Scout can knock his ball through the gates, the Sniper's Jarate splashes through the gate, and a few killtaunts can kill through it if both players are as close up to it as possible.
And if you shoot at just the right angle, the Pyro's flamethrower can burn enemies through the gate. This can and often will lead to a kill on enemies who react slowly or are taunting.
The Pyro's Rainblower taunt does an AOE rainbow (explosion) with a unicorn, and when it was first released, could kill through the gates.
In the first stage of Dustbowl, in the BLU spawn, it's possible to build a sentry between the spawn room border and the gate. This does a great job of clearing out the camping defenders.
When the Spy/Sniper update came out, the Spy could use his killtaunt while invisible (including the Dead Ringer, which made him almost invincible while doing it). This also suppressed the talking and his killtaunt is capable of killing several players or engineer buildings at once. This was fixed quickly, but he can still initiate the killtaunt and go invisible at the same time, which can catch people completely off guard when done right.
A rather memorable glitch came to light with fanmade server mods that let players equip hats they don't have. It was easily possible to trick the mod into letting you equip class-specific hats as another class, but the attachment point is completely off when not defined by the hat data, leading to Soldiers wearing hats on their crotch, Medics with hats floating high above them and similar hijinks.
With some custom server mods, it is possible to use a glitch to give one class another class's weapons, such as Heavy Weapons Guy with a Flamethrower. It's mostly harmless, and not very tactically useful, as only pure Spies can cloak and such.
There was also a glitch that allowed Spies to destroy their own sapper through the console. This lead to hilarity when done repeatedly, since the engineer would repeatedly yell "SPY SAPPIN' MY _____" and be spammed by sapper alerts. It also gave you a point for every one you built, which meant a lone dispenser could mean hundreds of points in one life. Unfortunately, it was patched after some time.
On some systems, spectating a Spy grants the alarming sight of left hand empty, right hand revolver: boom boom boom, reload with a third hand. (To reproduce this glitch in the first person view as a Spy: shoot a few bullets out of the revolver, bring out the cloak, and then reload. Cue mysterious arm popping out of nowhere to reload.) A Spy arm holding a revolver can appear on other classes' heads up displays as well.
The Pyro can use a weapon that deals extra damage against targets in mid-air, meant to be used in combos with the primary weapon's ability to push enemies into the air. This makes it very easy to abuse a glitch where if a Pyro airblasts an enemy down, they will still be considered airborne until they change altitudes by walking on an incline (which may not be close by) or jumping (and still take extra damage), a technique dubbed ground-stalling.
The Beggar's Bazooka brought a big heap of these. First, we need to understand how it works: When the fire button is held down, the Soldier starts loading rockets in the tube. He can then launch up to three rockets in quick succession when letting go, but if you don't, the Soldier will keep loading rockets, which will cause a misfire with each rocket over the cap, damaging the player. The misfire is rendered by the game as two rockets going off in front of the launcher. And here's where the bugs come in: First, there was a bug (which was already fixed) that made the game miscalculate where to set off the explosions, and ended up blowing up completely unrelated enemies. The second one (which is still active), is that even if the soldier is stunned, taunting, or in humiliation, he can keep loading rockets and even fire what he has loaded, which results in players recreating the Cow Mangler Kamehameha above, and overload kamikaze attacks during humiliation. Last (which will probably be left in because it isn't exactly a bug) but not least, the overload can be used to rocket jump even while airborne, which, combined with the gunboats and/or overheal, essentially lets the Soldier fly across the map in seconds and rain rocket salvos down on the enemy snipers. Yet another one involves the infinite clips and ammo mode, where the soldier with this out will uncontrollably startspewing out rockets.
And every so often, your heads up display will show the wrong team's colors, including what color you turn while ubering.
Also for now, Mann vs Machine has 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, still looking human despite apparently being robots.
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.
Though somewhat minor, there's a glitch with weapons like the various Rocket Launchers, Grenade Launchers, and shotguns where taunting at the right time during the reload animation can allow you to taunt while reloading. Not very useful normally, but if you're playing on a server that has tauntcrits...
A very memorable glitch in Doom 2 has the Arch-Viles, a Mook Maker who can resurrect dead enemies, creating "ghost monsters" if they try to revive monsters killed by being crushed (like with a Descending Ceiling). 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). 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 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.
And much more! The grand-daddy of good bad bugs in Doom is 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 to the direction you want to go, and strafe in the opposite direction 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.
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 Mook Maker 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. 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. 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 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.
That was a result of cartridge tilting, which is where you lift one side of the N64 cartridge out of the slot, screwing with the communication between the cartridge and N64. This can cause damage to either the game or the console, but common side effects are often amusing, including screwing up object/character part orientations, scrambling text, and garbling sound. Do a search on Youtube.
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.
Crysis features some extremely amusing bugs, but none are as awesome as the mighty Ascending Frog. By picking up a harmless frog from the ground and lofting it heavenwards, you will be awed as it proceeds into the stratosphere, unhindered by your puny gravity. Ascending Frogs can be seen in the wild in the cave immediately before the alien ship, where they cheerfully levitate, jumping vertically every time their hop animation cycles and staying that high until they hop again. Vicious brutes who shoot the noble Ascending Frog and subject it to gravity will pick the slain creature up and find it levitates above their hand, rotating slowly in righteous fury at their vile conduct and no doubt dubious sexual morals.
In BioShock, 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.
Even better, it is possible to spawn two extra Little Sisters, one in Port Neptune by killing the three "official" Big Daddies and harvesting/rescuing their Little Sisters before you go upstairs to the Wharfmaster's Office (tricky, but doable). Once you have collected all three Little Sisters, then you may proceed upstairs, where you can catch the extra Little Sister for bonus Adam. The second extra Little Sister is in Fort Frolic, and is a bit more complicated to catch. Once Sander Cohen opens up Poseidon Plaza for you, head straight to the Plaza but don't go upstairs until you rescue/harvest two Little Sisters. When you are finished, head upstairs to Rapture Records and finish your objectives (this is easier if you save Martin Finnegan for last). When you photograph Cobb, you should hear the third Big Daddy/Little Sister combo walking around outside. The key here is not to return to the Atrium and Sander's Masterpiece until all three Little Sisters have been found in Poseidon Plaza. When you do return to the Atrium, you should find the fourth Little Sister wandering around. An added extra is that if you are rescuing Sisters for Tenenbaum's gifts, the two glitch Sisters will count towards your total which means you can get gifts earlier in the game.
An extra extra is that, even if you're aiming to save all the little sisters to get the good ending / Tenembaum's gifts, you can harvest the Glitch Sisters (or save them and harvest two of the others) without incurring any bad karma.
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.
Late in the game some enemies will play dead to jump you as you walk past them, hiding among actually dead bodies. However, dead bodies can be picked up with telekinesis while the pretenders can't, so you can move all the dead ones and booby trap the others.
Also, your aiming reticle will turn red upon targeting a live splicer pretending to be dead while a real corpse will keep it white. A quick scan of the room will easily reveal any ambush.
When you save a little sister, you remove the Adam from her and turn her into a normal little girl. If you shoot her before she runs away, she will ignore being shot, and an amusing recycled message will appear:
The machine that you have damaged has been HACKED and is FRIENDLY to you.
The Wii version of Call of Duty: World at War has a few of these, mostly in the multiplayer levels. This can make surviving in these areas nigh impossible for those who don't know how to exploit them.
Occasionally on the mission "Blowtorch & Corkscrew" the enemies' automatic weapons won't fire. They'll just stand there and point them at you like idiots. (Their mounted MGs, grenades, bolt-actions, and bayonets are still threats though, as are any other explosives.)
Call of Duty 2, at least in "Repairing The Wire", allows you to destroy immobilized tanks simply by bashing them with your weapon. It's pretty much impossible to not die in the explosion yourself, however.
Several in Modern Warfare 2, though whether you thought they were good or not is subjective.
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 More Dakka champion of video gaming.
Another allowed players to "cook" a Javelin rocket, which would fire at their feet when they died, killing everyone in a 15-20 foot radius. [[Hottip:*: It was possible to survive. While most players who did this hid in corners where they coulden't be shot, using the riot shield and blast shield would allow a player to shield bash the anybody cooking and live.]]
Sometimes, players would be connected to a private, hacked game that would violate every rule of the game. At times, players could randomly find themselves in with seventeen other players in map designed for four, all with the aforementioned infinite ammo bug, the ability to jump 100 feet into the air, and gaining ludicrous amounts of experience with each kill. Not to mention, the point limits were set ludicrously high so the game would pretty much go on forever until you leave or the game crashes.
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.
Using Bling to attach both a shotgun and a suppressor to an assault rifle will cause the shotgun to also be affected by the suppressor - even though its sound is unchanged, firing it will not mark you on the enemies' minimap.
In the original Modern Warfare, during the finale shootout on the bridge, occasionally one of the AI Marines would get out of place and not die when scripted. When the Big Bad approaches you in slow motion, he attacks and kills him for you.
Survival mode in Modern Warfare 3 resets your sprint meter if you jump while sprinting. This essentially brings back a form of the classic bunny-hopping, with players jumping all over the place so they can sprint forever.
Perfect Dark has a notorious glitch in the very first level that can allow the player to skip entire sections of the level, and if one is playing on Agent, it is possible to finish the level in less than 7 seconds.
Halo: Combat Evolved: Hunters supposed to be one of the game's most fearsome enemies, can be killed with a single pistol shot to the back. This is because they're supposed to be weak in their soft orange parts, but the developers got lazy and coded all the orange as their head, meaning that one hit from a headshot capable weapon (like the pistol) will bring them down. Unfortunately, this was fixed in later installments.
In Halo 2, there is a bug where crouching and then attempting to stand in a low area causes the physics engine to confuse the player with the map. The player can exploit this by jumping and landing a line of the map's geometry in a particular way. When the player does, you are launched high into the air and often into an area that you couldn't reach any other way. Here is a detailed description of this glitch on a wiki devoted to Halo.
You could also harmlessly sword lunge at an enemy player by canceling the damage portion of the attack but not the lunge. You could do this repeatedly while underneath a jumping player to float up a wall. Additionally, button combinations could allow you to shoot an entire clip of the battle rifle at once or punch someone while shooting them, both resulting in an instant kill when done correctly. Amusingly enough, there was also a button glitch which allowed you to shoot yourself in the back of your head. It was more useful than it sounds.
Ladders in Half-Life 1 and its expansions work in odd ways. While on a ladder, looking horizontally while trying to walk toward it makes one ascend, and conversely trying to walk away makes one descend. On the other hand, looking up or down while pressing forward or backward makes one go in the expected direction. Put them together and interesting effects happen. For instance, looking down toward the ladder at a 45-degree angle and pressing forward results in no movement because the two rules conflict. More importantly, though, speedrunners can take advantage of the two: normally, the speed is limited to the maximum one could get with either rule, but if one, say, looks straight up and turns 90 degrees to the side while pressing both forward and the direction toward the ladder, the engine gets confused and applies both effects, resulting in a double-speed climb! (There's a risk of falling off if not turned exactly 90 degrees, though, due to the fact there is also a lateral component to ladder movement.)
In the same game, each door has a specified value for inflicting damage if the player is caught between a door and a wall. There is one door in "Surface Tension" which is given a negative value for damage, presumably because it opens away from the player and therefore should not be able to crush them. However, it is possible to get caught in the door in a way that inflicts damage — and if you do, it causes you to gain health at a rapid speed. And there is no cap on how much health you can have. By the time you decide to free yourself, you can be left with so much health that you might as well be invincible. The single-segment speedrun available here exploits this bug so that the player can use high-risk damage boosts whenever he wants for the rest of the game.
Some enemies (even Gargantuas) can be skipped without fight with some abuse of scripts (i.e. disabling the trigger).
In Half-Life 2 the player could hold a small flat object underneath themselves and jump repeatedly. Since the object is treated as a platform, and carried objects are held at a set distance, you could use this to hover over sizable parts of the level when done right. This has since been patched in Episode 1 and onwards, but is still a vital part of the "Half-Life 2 Done Quick" speedrun.
There's also a bug related to the Striders and Antlion Guards: the Antlion Guard has a charge attack that flings its target backwards. In the game itself, the Guard only fights Combine infantry, the PC, and (once) a Combine APC, which is specifically flagged as immune to the fling. Striders are not flagged as such, and spawning the two units together in Garry's Mod can result in Striders being hurled by Guard attacks.
Also, at one point in development, the Combine Gunship was intended to have a heavy beam cannon attack that hit targets directly beneath it. It was removed, but by the relatively simple expedient of ordering the AI of all Gunships spawned in the game not to use it (in case the devs wanted it back at some point). Supposedly, Gunships spawned via console command are not so restricted.
Another bug with the gunship is how they decide what to shoot at with their auto-turret. They're programmed to target what they consider the most dangerous threat they can see, and the player is supposed to be the biggest threat they can possibly see - what the devs hadn't expected was for the gunship's AI to decide that a rocket-propelled grenade fired by the player is an even bigger threat and actually try to shoot it down. This was left in to add to the challenge in fighting them, forcing players to take advantage of the RPG's guidance system by circling their rockets around and taking other odd paths to keep them from being shot down.
Half-Life 2's SMOD added tons of (hilarious) new weapons, more enemies to kill with said weapons, and a mighty melee attack for Gordon Freeman. When the Freelance Astronauts played the airboat levels with it, they discovered that you could use this powerful kick even while sailing... and it would propel your boat forward. Thus began the KickBoat Saga.
The original Deus Ex 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.
During the fight with 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 Jensen knocking him out with a single punch. Even on the hardest difficulty.
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 this4Player Podcast video.
The original Quake engine had a bug where movement speed could be increased significantly by so called "strafejumping" or "circlejumping" where the play would strafe and turn while jumping. Such movement, requiring some skill, could significantly boost the running speed above the intended (by 50% or more). The bug was kept in the Quake III engine as a feature, since it was by then thought of as a fun way for players to improve their skill beyond conventional tactics, through training of jump timing and muscle memory for mouse control. A vast number of "jump maps" and "trick maps" have been designed for Quake III. In the Quake III mod, Urban Terror, the concept has been extended to wall jumps (continuing a jump upwards by kicking a wall in the middle of a jump), ledge grabbing, and knee/crouch grinding, resulting in a whole jump/trick community, still meeting on jump servers and designing new maps as of 2009. Quake Live, which is basically a browser version of Quake III, features tutorials on strafe jumping.
In the original Thief the player could reach ludicrous speeds by "bunny hopping", or jumping repeatedly. This would cause the second jump to keep the momentum of the first, ad nauseam. Using this trick, the player could zoom by any guard. He could also kill himself by running into a wall or furniture at 200mph, or send himself into low orbit by bunny-hopping up stairs. Game physics are fun.
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. Taa DAA! 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!
Halo 3 had a variant on this where, if holding a heavy object, the player could walk diagonally and the normal slowdown in movement applied by holding heavy objects was negated completely.
Tobias Bruckner in Turok: Evolution became a byword for terrible final bosses when, after doing absolutely nothing for the entire span of the game, the genocidal cowboy antagonist who killed the eponymous Indian's tribe appears riding a T-Rex for no reason at all. 90% of the time he won't move unless the player moves closer to him, leading to a grand finale of unloading your gun into a completely stationary target.
Jedi Academy 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.
The iPhone port of Wolfenstein 3D has a tendency to slow to a chugging crawl when firing weapons after playing for a length of time. This makes boss battles a lot easier as you can rapidly shoot them faster they can hit you, which makes up for the added difficulty of it being near-impossible to strafe left and right.
Blood had a glitch where, on occasion, enemies who are killed by being set on fire would continue to run around in their on-fire animation indefinitely, unable to damage you but also invulnerable to everything you had short of explosives.
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 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 up the first SMG 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 for two reasons: 1) the command to just drop a weapon was removed, and 2) there's now a fourth weapon slot, so finding enough weapons to juggle around to take advantage of the glitch for just one of them is incredibly hard to do.
Red Faction had a glitch in multiplayer mode when the invincibility cheat was active; the flamethrower's alternate attack would set CPU-controlled players on fire, but never let them die. They would run around forever screaming in agony until you turned the cheat off. It was possible to inflict this cruelty on an entire level's worth of bots.
It's possible, on a certain deathmatch map with a large overhead walkway, to destroy the entire walkway. However, there's a spawn point on top of that walkway, with a pit beneath it. If you're unlucky enough to spawn there when the walkway has been destroyed, you spawn in midair and promptly fall to your doom.
In the manor house level in Medal of Honor: 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 Stealth Run 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 anyway as soon as it falls as far as it was supposed to.
Turok 2: Seeds of Evil, being made in the days before console online gaming, allowed one to access cheats during multiplayer mode. While this leads to interesting situations like infinite ammo missile launchers, the cheats also include a level skip function. This means that you and a friend can access the single-player levels, which effectively turns the game into a two-player co-operative game. In addition to that, you can play the multiplayer with just one player. This means you can play the single-player game as different characters with different speed and health settings, including the velociraptor,which can only use melee attacks.
There was a way to make the Witch harmless in Left 4 Dead 2 if you have a gas can and a propane tank or grenade launcher. When the Witch is hit by an explosion directly, she stumbles and then freaks out and attacks. However, if she stumbles and gets set on fire at the exact same time, the Witch would flail about as she burns to death while not attacking anyone since her AI still thinks it is in stumble mode. This can be a life saver on Expert and Realism.
Incendiary ammo can't set off car alarms, even at point blank range.
Because of how the navigation meshes are made (determines how the AI can move around the map), sometimes it isn'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.
There's also a few glitches regarding dialogue from the L4D survivors in the sequel:
Sometimes if Francis causes friendly fire damage to Bill, Bill's voice when he scolds Francis will come from Francis' model instead, making it look and sound like Francis is channeling Bill.
When Zoey heals someone other than Louis and they give her their thanks, Zoey may say "Anything for you, Louis."
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.
Killing Floor 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 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.
The 2012 Summer update seems to have created a glitch where, at least in singleplayer, players can carry nearly double their usual maximum weight capacity.
Counter-Strike players almost universally abuse the glitch of firing a bolt action rifle, then quick switching to the next weapon and back to avoid having to work the bolt.
The Postal 2 mod "A Week in Paradise" adds a katana, which has a secondary fire that allows you to throw it at people for a single long-range kill, after which you have to pick it back up. Attempting to throw it at someone who is right next to you causes it to duplicate, allowing you to keep your first sword while impaling the guy with a second one.
PAYDAY: The Heist has the Cloaker, a special SWAT unit that can instantly down you, who is quite buggy. Normally, the Cloaker is supposed to fire at you and chase you down should you attempt to run away. However, there are times where the Cloaker is just frozen in place and is oblivious to your presence, even if you're right next to him.
The Evil Smasher ("Evil will be SMASHED!!! WITH SMASHING!!! EVIL!!! SMASH!!! ER!!!") is a middlin'-meh gun itself, but has a special effect; when reloaded, it has a chance to gain accuracy, damage and a larger magazine size, until the next time it's reloaded. The effect is displayed by an explosion on the reload. It's supposed to apply the effect then remove it, leaving it with the temporary buff. If you interrupt the reload during the explosion by switching to a different weapon, it 'infects' all the slots, and now all weapons get the buff...which stacks and does not go away. Fire a shot, reload the weapon, and it gets more powerful, more accurate and a larger magazine. This video shows the Infinity pistol, which normally has a magazine size of 1 (it doesn't use ammo normally) with a mag size of 477, and shows him afterward taking down a raid boss that is normally a minutes-long fight in less than five seconds.
The ragdoll physics... don't quite work right sometimes, especially with Loaders, as shown here. Unlike the Evil Smasher bug, the devs simply left that one in just because it's hilarious.