Ascended Glitch

""It's not a bug, it's a feature.""

- Dennis Nedry, Jurassic Park (the novel)

When a Good Bad Bugs is particularly liked by fans of a game (more than just the God Modders), sometimes something interesting happens.

This is when a glitch is upgraded to the status of a legitimate gameplay mechanic or feature in a sequel, update, or expansion. This is a refreshing attitude from game developers, who otherwise tend to take a zero-tolerance approach to the squashing of bugs-- the exact opposite of the attitude usually taken by fans. However, some of these aren't actually glitches, they're exploits.

Of course, bugs that break the game balance usually aren't eligible for this, unless the game is rebalanced to accommodate them. Game Breaking Bugs are never eligible, of course, owing to making the game Unwinnable. No-one likes those bugs.

See also:
 * Good Bad Bugs, where glitches can be used for gaining an advantage of the game or just pure entertainment.
 * Ascended Meme, where those related to the source material of the meme recognize it.
 * Ascended Fanon, where fan-suggested ideas and stories are written into the Canon.
 * Throw It In, which is like Ascended Glitch, but happens when the work is in development stages.
 * Violation of Common Sense, which most of these bugs tend to rely upon.

Action Adventure

 * In The Legend of Zelda Links Awakening, there was an Easter Egg where pressing both buttons at once allowed you to fire a bomb attached to an arrow. While not necessary at any point in Link's Awakening's main quest, Bomb Arrows became an official game mechanic in The Legend of Zelda Twilight Princess.
 * In the original The Legend of Zelda, players can flip Link backwards while stabbing with the sword to attack both front and back. In A Link to the Past, this move became officially known as the spin attack, allowing Link to attack in a full circle, and became a staple since then.
 * The 3DS version of The Legend of Zelda Ocarina of Time contains many of the bugs from the original N64 release, such as the ability to enter certain areas that should be locked off to you. The original plan was to fix all these bugs, but many of the developers argued that the bugs were a part of how the game played and as such should be left in. In the end, any glitch that wasn't actively harmful was left in as a deliberate feature.
 * Jurassic Park the Game replicates the blooper from the original movie where the security camera feed on the computers is just a video file playing in a media player if you look closely enough to see the playback position bar at the bottom. The game has these on live security camera footage too.

Action Game

 * The combat system in Devil May Cry was born from a glitch that was removed during the development of Onimusha. It was a glitch where you could launch people into the air and juggle them. It got cut for being out of character for the game, but the glitch was so cool that Capcom decided to get some ideas from it.
 * Hotsuma's scarf in Shinobi for PS 2 was a victim of a programmer's prank where he made it ridiculously long. As it turned out, everyone thought it looked better that way, and they even made it longer before the game's release.
 * Lara Croft's breasts in Tomb Raider are actually an example. One of the programmers accidentally put an extra 0 in the size value, and the other programmers liked it, so they left it in.
 * In the NES game Ninja Gaiden, clever players could quickly unstick and restick to a wall to climb it up even if it had no ladder, a strategy vital to beating the game. The sequels added the wall climbing skill as a default ability and adapted the game levels around it.

Fighting Game

 * Combos in Street Fighter II, which would become a cornerstone mechanic of not just the Street Fighter series, but the entire fighting game genre.
 * Cancelling: Inputting a second command quickly before the character has finished performing the first one can lead to the second move occurring instantly, bypassing the recovery animation of the first. This technique is now the cornerstone of many fighting games, where it is possible to cancel attacks, throws, jumps and dashes into one another, to say nothing of regular combos.
 * Many combo-oriented games feature a variaton of "Super Cancel" which can cancel any move into any other at the cost of the special gauge.
 * Cross-ups: In games where blocking is achieved by holding "back", a cross-up is a situation where it is difficult to judge the correct direction to block your opponent's attack, such as when attacking while jumping right over the character. Needless to say that initially this was just a design oversight, but later it was polished and promoted to another aspect of mindgaming your opponent.
 * A graphical glitch in the early installments of Street Fighter II, Ryu and Ken would occasionally throw a red-colored Hadoken instead of the normal blue. Despite no evidence that the red Hadoken did anything special (and specific Word of God statements to the same effect), players insisted that the red Hadoken did extra damage (or was faster or gave an extra split second of impact recoil). In Super Street Fighter II, the red Hadoken did make it in as an actual special move for Ryu as the "Shakunetsu Hadoken": a more damaging Hadoken that took longer to throw.
 * The "invisible Dhalsim" glitch was later incorporated into Dhalsim's Yoga Teleport.
 * Just about every advanced tactic used in competitive Marvel VS Capcom 2 play is a glitch or caused by a glitch.
 * Cody's "Final Destruction" super move from Alpha 3. The move itself looks rather weird (if it connects, he will jab his opponent twice, turn around, turn back around, jab them twice again, and do it about four times before ending with a powerful string of attacks). However, it's actually a Shout Out to a glitch from Final Fight where he could do this to destroy his opponents with ease. If used in X-ISM, this move changes all his attacks into his Final Fight attack strings instead.
 * A couple of bugs in The King of Fighters made their way into canon. One of them is Leona's respect for Chang (she salutes him starting with KOF98; it cued some nice Wild Mass Guessing back in the day). Another one is Kim's midair super in KOF94; it was definitely a bug that affected other characters with midair command moves in KOF94 and KOF95, but Kim's ability to do the Ho-o-kyaku in midair has been an intentional ability in many later games.
 * Mortal Kombat:
 * Baraka's "whirling dervish" move was originally the result of a glitch, but developers liked it so much, that it was eventually added to his repetoire.
 * There was a glitch where Johnny Cage's uppercut fatality would decapitate the opponent twice. Developers took this a step further in II, giving him his hidden triple decapitation fatality.
 * Speaking of palette swaps, there's Scarlet.
 * Noob Saibot was Dummied Out of the arcade version of Mortal Kombat 4, but made unlockable (although he lacked fatalities) in the console versions, and had a couple rather glitchy alternate costumes.
 * The legendary "Ermac" glitch, however, never happened, though it became Ascended Fanon in later games.

First-Person Shooter

 * Quake had the Strafe-jumping, which was a bug at the time, but became so popular that it was later the trademark of its multiplayer component. An uproar went through the community as the mechanic was removed in one patch of Quake III Arena, so a new patch featured it as... well... a feature, which was part of its success, along with many other stuff. Quake Live even has a tutorial with many courses which rely on this.
 * Taken to a ridiculous extreme in Warsow, wherein the game engine was built from the ground up to accommodate movement tricks that started as glitches in Quake.
 * There's also the "Rocket Jump". Point the rocket launcher straight down, jump, and fire immediately after the start. The player will take an assload of damage, but (s)he'll also be flung much farther than otherwise possible by jumping. When exactly this ascended is unclear, as the third episode of Doom had a secret exit that was designed to only be accessible by launching yourself sideways off a wall with a rocket (other ways have since been found), and in Quake II, there's a secret spot which can be only got to by rocket jumping. In the latter, when you land, the text appears: "You crazy rocket jumpers!"
 * One of the the single player maps in Quake had a secret area accessed by a teleporter hanging from the wall. In order to reach the teleporter, you had to fire a grenade into a small hole in the ground beneath it, and jump over it just as it explodes. So id did have some knowledge of this technique in mind when designing the game.
 * The Expansion Pack Quake II: The Reckoning mocked it in one of their levels. In it, you would see an Invulnerability which vanished after a certain distance. A message would show up later saying "No prize for you, rocketman." The expansion is also infamous for having increased the Rocket Jump damage and distance very much...
 * Marathon actually had grenade jumping similar to "modern" rocket jumping, about the same time as Doom. It became an ascended glitch, too, with secret areas requiring it. In lower-gravity stages, you could even use the flamethrower as a makeshift jetpack. Although you could use the recoil from the rocket launcher to propel yourself, trying to use a point-blank blast to launch you upward would instead kill you instantly. The recoil physics were nerfed in the second game's engine, precluding the "jetpack flamethrower".
 * Team Fortress 2's Soldier was designed with rocket jumping in mind, and can acquire boots that, in exchange for losing a shotgun, improve his rocket jumping ability. The "Gunboats" reduce damage taken from rocket jumps, and the "Mantreads" turn rocket jumps into an attack.
 * Several Valve games have this:
 * In the update that added "King of the Hill" to Team Fortress 2, when the round went into Overtime, the announcer would repeat her proclamation of "Overtime!" over and over until the end of the round. This one was so popular that when Valve fixed it in the next update after the patch, they added a server option that allowed admins to keep the buggy speech if they so desire.
 * Further, an October 13th, 2011 patch added a server option to re-enable a popular bug, "Taunt Switching" (where the model for one weapon replaces another in a taunt, so that, for example, a Spy could pretend to fence with a revolver, or a Demoman could take a drink from his grenade launcher).
 * The Sniper Vs. Spy update canonized the disguise kit+ crouch+ look up "Spy Crab" pose (in which the spy's model stretches beyond his animation skeleton) by having approximately 1 out of 10 taunts with the disguise kit use an alternate animation with the spy dropping into a crouch and making claw motions with his hands.
 * The Spy class itself was inspired by a bug in the original Team Fortress, where players would sometimes see other players with the wrong team's color.
 * Before the Uber Update, there was an exploit for Demoman to allow him to turn sharply with the Chargin' Targe; this exploit was later turned into an item called "Ali Baba's Wee Booties". The only downside is that you don't have a Grenade Launcher.
 * Early in Half-Life 2: Episode One, Alyx asks her robotic companion dog "You did do the math, right?", right before being flung across a bottomless pit into the citadel. DOG originally wasn't supposed to do anything, but a script error caused DOG to insert his head-shaking animation, right after that line, and the playtesters loved it so much it was worked into the actual script.
 * Similarly, Episode Two had a bug in the physics engine where if the player tossed in a grenade in a dumpster, the grenade would be tossed back out as if someone was in there and chucked it back. They liked it so much that it was kept in.
 * There's actually a fast zombie in the dumpster, which adds to the hilarity. It can't see the player and doesn't make much noise when idle, but once the player gets close enough for it to hear footsteps, it jumps out.
 * The Combine gunships in Half-Life 2 had an AI glitch during testing which caused them to shoot at rockets fired from the laser-guided rocket launchers, forcing playtesters to guide missiles in all manner of odd paths in order to confuse the AI and hit the vehicle. This proved to be so popular that it was left in as a feature.
 * In a similar act of serendipity that ended up making the game more interesting, during the airboat vs. helicopter battle, the "mine spam" dropped on the player was originally due to one of the programmers accidentally making the helicopter drop mines instead of shooting bullets.
 * One sequence in Left 4 Dead 2 involves making your way through an abandoned sugar mill. An odd glitch caused a larger-than-usual number of Witches to spawn in the building; the testers thought this was a stroke of genius, so Valve left the glitch in and wrote the Witch's fondness for the smell of sugar into the backstory.
 * A bug with jumping inertia in Starsiege: Tribes let skilled players reliably "ski" downhill at high speed to build up speed. When it was fixed, not only after player feedback was it quickly put in, skiing became a trademark feature of the sequels, even being required at various points in Vengeance's campaign.
 * Lampshaded by this April Fools' Day post, which claims to have "fixed" the bug for Ascend even though by this point it was an absolutely critical gameplay mechanic.
 * Halo had a bug in which, when the player looked all the way down, the character model's head would instead look straight forward. This got used heavily by machinimators (most notably Red vs. Blue), since it looked as though the character was at ease, rather than constantly pointing his gun at other people. For the sequels, Bungie fixed the glitch, but added a feature that allowed a character to put his gun at a "rest" position.
 * In Halo 3's map editor Forge, people used glitches and tricks in order to place objects in order to make cool new maps. In Halo Reach Bungie changed these tricks into features of the new map editor.
 * Possibly inspired by the Honor Guard Councillor glitch in Halo 2, one of the two types of hidden "BOB" Elites in Halo Reach is white with a random armor type.
 * Among other things, Painkiller pays homage to the traditional quirks in FPS physics and some secrets are hidden in ways that require you to take advantage of these. You need to be able to exploit the way the hitbox reacts to almost invisible protrusions in walls to climb them with constant jumping (Asylum). You must at times resort to bunnyhopping (even in circles!) in order to gain enough momentum to leap somewhere else (Castle, Colosseum). You must remember that doing an U-turn in midair is a perfectly plausible thing to do (City on Water). Even rocket jumping has its own hotkey.
 * Any number of weird bugs have been adopted by the Doom community in order to add bits of interest to their custom levels. Among these: making floors look like water, making enemies nearly invincible, and making "voodoo dolls" of the player character whose deaths will also kill the player.
 * The voodoo dolls were even used in Final Doom.

Four X

 * In X3: Reunion, ejecting your shield generators then scooping them back up instantly recharges them. Egosoft kept it in X3: Terran Conflict because the trick is useless in combat (you have to stop maneuvering and back up in order to scoop up things you ejected from the cargo bay), and nobody wants to wait half an hour for their heavy capitals' shields to recharge on their own.

Hack and Slash

 * Diablo II's Hammerdins. Basically, Blessed Hammer is a nigh-useless spell on the non-spellcaster Paladin. Due to some bug, Blessed Hammer's damage was boosted by the Concentration aura (which should only be boosting physical attacks). This created the Hammerdin, making a Blessed Hammer/Concentration combo a viable character build (with the right equipment, of course). Blizzard made sure it would continue to work properly in subsequent patches.
 * Incidentally, this made Hammerdins into one of the strongest builds in the game (some would argue the strongest) and is a huge gamebreaker.
 * In Magicka, the Teleport spell is normally only obtainable at the beginning of level 7, but it can be gotten (in multiplayer) in level 1 by laying mines at your feet and blowing yourself up and over a wall. The developers thought this was so clever they opted to leave it in.

MMORPGs

 * The Corrupted Blood incident in World of Warcraft later inspired an actual in-game event, the Plague Outbreak.
 * The bank robbery/mayhem missions in City of Villains were originally intended to have the destructible environment scale with the player, however on initial release all the objects were unintentionally locked at fixed, low, levels. This bug became wildly popular because players now had a way of showing their characters becoming more powerful: the car that took forever to destroy at low levels disintegrated with a single attack at a higher level, and area of effect attacks caused massive destruction. The development team realized they'd accidentally hit on a good thing and have left it in.
 * When swimsuits were introduced as equipment in Phantasy Star Universe, they took up all three clothing slots. A bug allowed characters to equip two layers of clothing at once, which was mostly pointless, but allowed a few would-be fashionistas to wear a swimsuit top with normal pants (or, if their character was male, go shirtless with normal pants.) A later patch made swimsuit tops, bottoms, and sandals separate items, making the mix-and-match easier.
 * In Runescape, certain members-only items of clothing were made available. It was discovered that some of those items ([[http://runescape.wikia.com/wiki/Gloves_
 * An extremely early example (2001 or so) would be the now taken-for-granted feature that makes items only visible to the player who dropped them for about a minute before being visible to everyone else. It was originally a bug, but after it was removed, public demand brought it back.

Platform Game

 * Reportedly, the blocks from Super Mario Bros that release multiple coins when hit multiple times were actually due to a programming mistake. After fixing the blocks, the designers decided they missed them and deliberately programmed them back in, and they've been mainstays in the series ever since.
 * Jumping against a wall in the original Super Mario Bros with frame-perfect timing would allow one to execute a Wall Jump. Mario gained this ability for real in Super Mario 64.
 * Not quite a gameplay mechanic, but a nod to another glitch - When Mario arrives in the Underwhere in Super Paper Mario, a Shayde tells him that some call it "World -1".
 * Also in Super Paper Mario, pinning a Koopa shell against a wall and continually jumping on it would (eventually) make you lose points, a nod to the classic Infinite One Ups instance.
 * Speaking of such, Infinite One Ups were included in both New Super Mario Bros. games, to the point that one of the Hint Movies in New Super Mario Bros. Wii shows Mario executing the technique (the World 2-3 Infinite 1-Ups video, to be exact).
 * Heck, the Infinite One Ups bug was so popular in Super Mario Bros that the designers made the glitch possible to do in the very beginning of World 1-1 in the difficult Japanese version of Super Mario Bros. 2
 * Again, also in Super Paper Mario, one of the Sammer Guys is named "Over the Flagpole" and talks about cheating. This is regarding a glitch in Super Mario Bros. where you... well, go over the flagpole.
 * And in the Japanese SMB 2, some warp zones were hidden beyond the flagpole; however, some of these warped you backwards.
 * Though less of a bug and more of just an oddity, holding an item in Super Mario World made the swimming physics change drastically, inexplicably giving the player constant forward momentum and allowing them to swim down instead of up. This was nodded to in Super Mario 64 and Super Mario Galaxy, as holding a Koopa shell underwater acted like an underwater jet ski and constantly propelled the player forward.
 * Super Metroid became exceedingly well-known (and loved) for speedrunning and Self Imposed Challenges based on utilizing glitches and circumventing the designers' intentions in the level design. In retaliation, the developers were dead sure to keep the player on a fixed path in the sequel, Metroid Fusion. This caused such an uproar among fans that, for the next game in the series, the developers slid all the way to the other end of the scale and designed the game around giving the player unprecedented freedom. Metroid Zero Mission featured: rewards for beating the game with minimal items (a classic self-imposed challenge), secret passages which allowed the skipping of "necessary" items, the ability to bomb jump and Wall Jump indefinitely, and the ability to use a charged shinespark in morph ball mode (based on a glitch in Super Metroid dubbed by fans as the "mockball").
 * A large amount of Artificial Stupidity and other tricks requiring exploitations of the NES version of Lode Runner made an appearance in Championship Lode Runner, which requires those tricks to be exploited. That and the difficulty of this game made it feel similar to some of the very hard ROM hacks.
 * In the first Chip 'n Dale Rescue Rangers for the NES, on rare occasions thrown blocks would fly in a zigzag. It became a chargeable attack in the sequel.
 * The Famicom Disk System version of Castlevania 1986 had a glitch that allowed Simon to keep climbing the famous staircase in the final level past where it ends. This became the route to an actual secret in Castlevania Rondo of Blood and Castlevania Symphony of the Night.

Puzzle Game

 * In the original Tetris, the maneuver known as a was a bug. It has been included as a feature in all subsequent remakes, and is considered a basic move by Tetris tournament players. It has even been expanded with "double t-spins" and "triple t-spins".
 * The first game of the Puzzle League series, Panel de Pon/Tetris Attack!, had a few bugs that added to rather than detracted from the tactical depth of the game. Due to the game's cult following and highly technical nature, later releases that recoded the engine manually added these bugs as normal gameplay features. Now, the series' lesson mode even teaches the player how to use them.

Real Time Strategy

 * War Craft 2 had a bug that allowed a player to get 100 extra lumber at the start of any game; this was so liked (due to allowing games to get going more quickly) that it became standard tournament practice to use it every game. When Blizzard remade the Battle.net version of the game, they fixed the bug but also had it start each player off with 100 extra lumber.
 * Blizzard intends to recreate Mutalisk stacking in Starcraft II, where mutalisks and other air units would be stackable given the right conditions, helping them become harder to focus down.
 * They have acknowledged it as far back as Brood War -- rather than get rid of it, they instead added new units to each race whose sole purpose is to deal with stacked air units.
 * It's not the only one either. They seem to have intentionally included every interface bug that ever became an element of strategy in the pro circuit, including a few parts of the dodgy unit AI.
 * The Salvage Corvette from Homeworld was only supposed to work on enemy ships that had been heavily damaged, but thanks to a programming error it ended up capable of capturing any ship. It didn't hurt gameplay balance in single-player and the multiplayer community loved it, so it was left in, though it was fixed in Cataclysm and HW2 went with a different mechanic altogether.
 * Many, many features in Defense of the Ancients. Animation cancelling. Killing your own creeps to deny experience and money to the enemy. Creep stacking -- luring a forest monster so far away from its spawn point that another one spawns in its place, then letting it walk back, creating a "stack" of monsters which you can proceed to farm very quickly with area effect spells. Orbwalking, a form of animation cancelling that enables one to shoot while moving, only for heroes with abilities that enhance one's normal attack though. You can give an attack order to an enemy at the opposite side of the map and it will aggro nearby enemy lane creeps so they attack you instead of your lane creeps, which helps push the lane a little bit without stealing last hits and money from the carry. Conversely, you can use your hero and other controllable units to block incoming creeps so they meet the enemy creeps closer to your tower, enabling you to farm more easily. Even individual bugs like the ability to angle a hookshot after it is fired are intentionally kept intact because they increase the skill level of the game.
 * And faithfully ported into both Heroes of Newerth and Dota 2.
 * In Startopia, the Polvakian Gem Slugs have a nasty habit of getting 'stuck' inside the mud baths of the Slugpartments and dying from kidney failure. The developers caught the bug early but left it in, finding it entirely in-character for Polvakians to get too obsessed with their own comfort to care about their own well-being and essentially committing suicide by luxury. Given the price of the Slugpartments and the level of luxury you need to attract Polvakians in the first place, the death compensation (1000e) is a paltry sum anyway.

Rhythm Game

 * The song "GAMBOL" in Beatmania IIDX has one of the easiest charts in the game; however a bizarre bug caused it to have much tighter timing windows than any other song in the game, making it easy to clear but extremely hard to score well on it. On Happy Sky, the bug was finally fixed: a version with correct timing was placed on the Normal difficulty, while the glitched version was moved to Hyper.
 * However, on the PS2 version of IIDX 11: RED, Konami decided to troll its fans further by giving the song an Another chart: which is exactly like the other two difficulties, except with the timing windows shrunk to the point where it's pretty much impossible to get any decent score on it!
 * Then on the PS 2 version of DJ Troopers, some secret codes were introduced (GJH and GJA, entered by highlighting a song starting with each letter and changing the difficulty) that allow the Gambol timing windows to be played on any song!
 * In Bit.Trip Beat, hitting the pong ball with the corners of Player 2's paddle will cause the ball to gain way too much momentum and go haywire, making the final boss trivial to beat. When the game was patched, the developers specifically didn't fix the bug because "it was a cool advanced technique".
 * Step Mania (especially 3.9 and "3.95" - the version used for In the Groove 2) had the infamous "negative BPM" bug which could be exploited to cause "warps" in a chart, which could be used for all sorts of interesting effects. The 4.0 branch unfortunately fixed this bug, but the fork sm-ssc (later merged back in as "StepMania 5") adds a new element called a "Warp" (along with "fake" arrows) which can be used for emulating this behavior in a more future-proof manner.

Role Playing Game

 * Final Fantasy I's "Peninsula of Power", while technically a glitch, was popular enough to have remained in all subsequent remakes of the game.
 * It's not really a glitch, but rather that they didn't quite get the map right. There's actually a similar spot in Final Fantasy IV (north of Mt. Ordeals) but it's rather more trouble than it's worth to use.
 * Similarly, the Critical Hit Glitch. In the original NES release, each weapon had a dedicated critical hit rate in its code, but the combat engine would use the weapon's index number as the critical hit rate instead of the intended critical rate. For example, Thor's Hammer was intended to have a crit rate of 0.5%, but in practice had a crit rate of 18%. For whatever reason, the developers have chosen to keep this error in all remakes of the game.
 * There is some fan speculation that the Pokémon Deoxys may be based on Missingno. The ridiculously high Attack and ridiculously low Defense of its Normal Form would appear to corroborate this, but there has been no official word on whether or not this is the case.
 * Considering that the (intentionally placed) Generation III glitch Pokémon have the exact same stats as Deoxys, this theory is fairly plausible.
 * Also, in Red and Blue, there was a truck found only when surfing before the S.S. Anne had set sail. A player could get to it through a glitch or by losing to a trainer after receiving Cut so that the ship wouldn't leave. It was intentionally kept in for the remakes. In both cases, the truck didn't do anything, but in the remakes, you could find a Lava Cookie nearby.
 * Final Fantasy VI is an interesting case. The well-known Vanish-Doom/X-Zone exploit wasn't fixed in the PSX port -- but the programmers DID change one boss to be invulnerable to Vanish (Phunbaba), sort of turning it legit for other bosses. (That said, Phunbaba could crash the game when being Vanish-Doomed, and they didn't even get the "invulnerable to vanish" part quite right.) However, the Game Boy Advance version of the game thoroughly squashed Vanish-Death/Banish (as the instant-death spells were retranslated), at least on anything immune to Death/Banish under normal circumstances.
 * In Super Robot Wars, all other music being overrid- TROMBE!
 * To explain this in greater detail, whenever Elzam Branstein would use an attack or do something that triggered his theme in the first game he appeared in, his theme would override anything that was playing in the background, even final boss music. The fandom found this hilarious, due in no small part to his theme being awesome, and Banpresto has deliberately included it in every SRW game where he appears since. It even overrides Komm, susser tod in the End of Evangelion route from Alpha 3.
 * It doesn't, however, override Source Music, such as The Beautiful Blue Danube being blasted from a battleship's speaker system, or the protagonist of Macross 7 performing a literal Autobots Rock Out session. Somehow, this just makes it more awesome.
 * Smith is a talking horse in the main Ultima series and Ultima: Worlds of Adventure 2: Martian Dreams. Originally, Smith was included in Ultima IV: Quest of the Avatar and was to give a clue on how to beat that game, but the programmers forgot to add it into his conversation tree. In Ultima V: Warriors of Destiny, Smith was given back his full dialogue and, up to Ultima VII Part II: Serpent Isle (were he almost gives a clue about the actual game), Smith has been giving out clues in the form of untimely information -- he tells the player what they should have done to have beaten the previous game, not the current one.
 * A very interesting pseudo-example in Baldurs Gate: Whenever a key character in the first game was missing from a scene (which could happen because of glitches), they'd be automatically replaced by a debug character called "Biff the Understudy". In Baldur's Gate II, Biff became an actual character with his own subplot.
 * In the original Super Famicom version of Final Fantasy VI, a bug rendered any item as being equippable as a helmet, with Edgar's drill tool being the best option. Dissidia Final Fantasy, already rife with Mythology Gags, saw fit to include a drill as being the headset equip for the 'Machine' equipment set.
 * In Improbable Island, at the start of a new day it gives you a message saying. "It is a new day! Strap your to your back and head out for adventure!", but if you had no weapon it would just be fists instead. This was removed in one update, but quickly put back in due to popular demand. It now appears on the merchandise.
 * The Elder Scrolls V Skyrim has a few physics quirks, one of which causes a player, companion, or random target to fly hundreds of feet into the air after being hit with with a Giant's club... in classic Blown Across The Room fashion. Though unintended, designers decided not to patch it due to its popularity with players.
 * In "Fall of The Space Core", a semi-official mod that Valve made to commemorate the opening of the Skyrim Steam Workshop, the Space Core can be returned back to space by being within the impact radius of a giant's club swing. It screams "SPAAAAAAAAAAAACE" on the way back up.
 * A glitch in the Paragon resolution of the Conrad Verner side quest in Mass Effect (where Shepard gently talks Verner into going home) causes the Old Save Bonus for Mass Effect 2 to assume you went with the Renegade resolution (where Shepard threatens Verner into giving up on becoming a Spectre). In Mass Effect 3, you can encounter Verner on the Citadel, where he apologizes to Paragon Shepard for claiming they threatened him with a gun--he was under a lot of stress.

Shoot'Em Up

 * Anyone who has ever played Space Invaders will recall that the enemy ships slowly speed up as you destroy more of them. This was due to an oversight by the original programmer - the presence of so many sprites loaded down the processor, slowing the game down considerably. As the ships were destroyed, the game had less processing to do, so it ran faster. He liked it so much that he left it in, and every Space Invaders game since has done the same.
 * The significance of this oversight extends further than merely subsequent Space Invaders games -- it introduced the entire concept of difficulty curves to video games.
 * The egregiously Nintendo Hard Gradius III AC had significant slowdown when there was a lot of enemies/bullets on the screen, which was practically required to navigate these bullet storms. The PS 2 Compilation Rerelease even had an option to emulate the lag/slowdown.
 * Similar to the Street Fighter II and Space Invaders examples, this would influence other games, as intentional, hardcoded slowdown when bullet density reaches a certain point has become a feature in many other games (particularly Bullet Hell games).
 * It got so widespread that some shmup fans have declared this "The ancient Japanese technique of slowing down time by creating more ****."

Simulation Game

 * In the original Zoo Tycoon game, there was a rather odd glitch in which an emperor penguin in a proper exhibit (for penguins anyway) would kill any other non-penguin herbivore added to the said exhibit. In the sequel, this is fixed; however, messing too much in the genetic lab minigame that comes with a expansion pack will result in a killer penguin. A psychotic rockhopper penguin with teeth, red glowing eyes, and a taste for dinosaur meat; placing any other animal (or a particularly annoying guest) in the same exhibit as the creature will result in a untimely demise, even a T-Rex. And you can't sell it. EVER. If someone offers you a penguin to adopt DON'T accept!

Sports Game

 * In Tiger Woods PGA Tour '08, there was a glitch nicknamed the "Jesus shot" which meant that in the right conditions, a ball hit into the water could still be played, complete with the golfer walking on water to hit it. Prior to the release of the 2009 edition, EA responded with a live-action recreation of the shot, claiming "It's not a glitch. He's just that good." Then, in the 2010 edition, the first bonus challenge was to recreate the scene, earning the achievement "Levinator25", named after the person who discovered the glitch.

Third-Person Shooter

 * In MAIET Entertainment's GunZ: The Duel, play is based almost entirely around sets of techniques mostly dubbed "K-Style" (with many subsets). By abusing many glitches in the engine, players were able to cancel portions of character animations; with proper timing, this allowed players to climb walls with excellent maneuverability, circumvent the cool-down on melee attacks, fire some guns far faster than their normal fire rate (creating automatic shotguns), speed across the map, and pretty much allow people to combo together many different things in short amounts of time. The developers opted to leave it in the game, as it was the cornerstone of Gunz's gameplay.
 * It's so beloved it's become an official feature in Gunz: The Duel 2.
 * Uber Entertainment's Super Monday Night Combat has Rampage Jumping. One of the playable characters, Cheston, was capable of activating a skill that caused him to pound the ground in mid-air, causing him to rocket forward quickly. The developers liked it enough that instead of removing the glitch, they added additional skill drain when R Jing to balance it out.

Wide Open Sandbox

 * Crackdown sported an Agency SUV which could not only jump with super-hydraulics at the highest level, but also drive up walls. According to the dev team, this function was created completely by accident, but was left in the final game since it wasn't truly a gamebreaker.
 * The DLC Expansion Pack includes a Dune Buggy that is especially designed for wallclimbing.
 * In Minecraft, the creeper's model was that of a failed pig model. Oh how far they've come.
 * The PC game Spore includes some glitches that have been left in, such as the ability to create invisible limbs and therefore make parts on the created creature float in midair. Another glitch made it possible to create asymmetrical creatures, and a later patch upgraded this to an actual feature, with a simple keybind (A to be exact) to get the previously tedious effect. Though, asymmetrical ships, vehicles and outfits tend to look better than creatures themselves.
 * A few month ago an update added the Captain's Quarters to Eve Online, which include various virtual screens bombarding one's avatar with videos. Some nifty people located these videos in the game folder and fiddled with them a bit, to change them to videos they liked. Recently, CCP made it very easy to do so without needing to change any of the game data.

Non-video game examples:

 * There's a web-enabled tool for creating fonts called Fontstruct. There was a glitch that enabled you to stack bricks you use to create fonts. In a recent update they made it an actual feature.
 * Traditional automatic transmissions using a fluid coupling and planetary gearset "creep" when in gear and not held against the brake. Continuously variable and dual-clutch automated manual transmissions don't have to, but are designed to anyway since that's what drivers expect in a two-pedal transmission system.
 * A glitch on the adoptables site Uni Creatures replaced the sprite for Flarius (a standard fire-breathing dragon) with that of Caprine (a goat). Shortly afterwards, the mods released a new creature: the goat-dragon Capricious.