The classic Space Invaders gradual speed-up of the invaders, as more are shot, was a Good Bad Bug resulting from the programmer pushing late 1970s hardware to its limit. It allowed the game to gradually get harder, creating the idea of difficulty curves.
One such example that's not an Ascended Glitch but a Shout-Out: I Wanna Be the Guy has a bug that makes it possible to skip Kraidgief's second phase before he rises up, which would also make the rest of the battle a total pushover since he would be at a much lower height and thus much easier to shoot. Word of God says it was kept in "since it works just like Kraid from Super Metroid".
Tribes's uniqueness was born of this — in the first game, "skiing" was a glitch based on clever usage of the jetpack to go really damn fast. It became the series's calling card, and in its revitalization Tribes: Ascend, it's not only a game mechanic, but THE game mechanic — the new user guide demonstrates walking, shooting, jetpacking, and skiing. You only pass if you can ski moderately well. The game is advertised as the fastest modern shooter.
When creating the character model for Tomb Raider's Lara Croft, creator Toby Gard was trying to make a minor adjustment to Lara's breast size. He selected all vertices in the breast to resize them, but the mouse slipped and they increased 150%. The team loved the new look and Lara's famous physique was born.
The ridiculously long scarf worn by Hotsuma in the PS2Shinobi is a result of a programmer gag: originally it was a normal-length one, but someone made it a lot longer, everyone liked it and they proceeded to make it even longer.
Knights of the Old Republic has an in-universe example: soon after Revan built HK-47, Malak asked it what it thought of him, and it responded by referring to him as a "meatbag". Malak was really pissed, but Revan thought it was so funny that he reprogrammed HK-47 to refer to all organics as meatbags.
Ocelot's trademark hand gesture was an improv by the motion capture actor that made everyone laugh so hard it became the character's Catch Phrase.
For Metal Gear Solid 3, Hideo Kojima had the motion capture actors for Naked Snake switched so that the one who specialized in action sequences did the dialogue scenes, and vice versa. In one scene, EVA leans in to kiss Snake, and Snake's actor, not used to doing romantic scenes, nervously edged away. They kept it in since the response was cute and fit Snake's character perfectly.
In fact, according to the director's commentary, a lot of the things in Metal Gear Solid 3 were improvised and not actually in the original script.
In a similar vein as above, the Honest Johns character of Metal Gear Solid 4's pet monkey was thrown in when they realized that Raiden's motion capture actor did a pretty good imitation of a monkey. That, as well as the fact that Drebin seemed too bland by himself in Kojima's eyes.
Kirby as we all know him was only a placeholder character for Kirby's Dreamland. They ultimately went with the little blob.
Wii Sports was originally set up to be a Mario title. The test audience surprisingly preferred the Miis.
One of the levels of Portal can be finished in mere seconds, by using shortcuts to skip the entire level. When playtesters figured this out, the developers decided to keep it, since only advanced players would know how to utilize it. In fact, in the advanced versions of that chamber, the qualifications for gold medals require that you do this. The commentary put it like this: If the exploit takes more skill and ingenuity to perform than doing it the intended way, it's perfectly reasonable to keep it in.
For anyone wondering why the Space Sphere is the only corrupted core to speak after being attached to Wheatley in Portal 2, it was thanks to Steven Merchant's outburst after hearing Nolan North recording the lines for the Space Sphere. The developers thought it was hilarious and just had to find a place to put it in.
Space Sphere: Gotta go to space! Yeah! Gotta go to space!
Wheatley: NOBODY'S GOING TO SPACE, MATE!!!
In a non-gameplay example, many of Raz's lines in Psychonauts were ad-libbed. They were so funny, the folks at Double Fine decided to keep them.
According to Dave Grossman Guybrush Threepwood's first name came when Monkey Island was in development. Since he had no name yet, his character art was simply named "Guy", and when Steve Purcell, the artist responsible for "Guy's" sprite, saved the file, he added "brush" to the end, to indicate that it was the Deluxe Paint brush for Guy, creating a file called "guybrush.bbm". They eventually just went with that, and had a great deal of fun commentingon his weirdname.
According to Tim Schaefer, most of the dialogue in Secret of Monkey Island was thought by him and the other writer to be placeholder, and that the bigwigs were going to hire some "real" dialogue writers to do the actual stuff, but the humorous placeholder dialogue was so funny that they decided to leave it in. This actually had quite an impact on the gameplay itself and lead to the "rubber chicken with a pulley in the middle" bit.
The name of the god Armok was taken from a variable used in the game's predecessor Slaves to Armok: God of Blood standing for the number of available arms: arm ok.
Working on the necromancy implementation for DF 2012, Toady discovered that he could accidentally cause necromancers to raise undead from skins, hair, and other waste tissue, in addition to more conventional undead. He decided this made as much sense as the walking skeletons did, and published it in his development log as one of the new features.
The "Prelude" (a.k.a. the "Crystal Theme") is quite possibly the most iconic song in the series. It was put together by Nobuo Uematsu in ten minutes.
Most of Kefka's dialogue in Dissidia: Final Fantasy was actually ad-libbed by his voice actor, Shigeru Chiba, and his tendency to change the tone of his voice mid-sentence was also due to Chiba.
Similarly, in Final Fantasy VI, the game that Kefka originated from, his infamous introduction scene (barring the flashback) was not in the original script. Originally, he was intended to simply arrive at Figaro Castle and demand for Terra. However, Yoshinori Kitase felt that the scene was too boring to make completely normal, especially when they had only Amano's artwork of Kefka to work with, so he added in a scene where Kefka is complaining about Gestahl sending him off to Figaro, in the desert, on a reconaissance mission, and then stating that there is sand on his boots, causing the soldiers to wipe the sand off, and then, after laughing maniacally, berates them as idiots, with the intention of giving the implication that Kefka may be missing a screw or two from his head. They kept it in, and this also resulted in Kefka's characterization that we know him by to this day.
On the subject of Kefka: "Dancing Mad", Kefka's final boss theme for which Laughing Mad was named. Not so much that the song wasn't meant to be, but how much there ended up being of it, was a complete throw-in by Uematsu, as revealed here:
Well, usually when you make a song it's two to three minutes in length, you have the introduction, the main part and the ending. But... for 'Dancing Mad' I didn't really put a stop on it, so I kept on working on it, working on it, working on it and that really let the song... you know... I got to play around with it for something like fourteen minutes, and it's really one of my favorites.
An in-universe example of this trope was also used. Ultros, feeling sore about his earlier defeat, decides to get his revenge by dropping a 4 ton anvil on top of Maria/Celes, but had a five minute delay due to his miscalculating the amount of strength and time he needs to actually attempt to push it onto Celes/Maria, giving Locke and the rest of the Returners (who discovered Ultros' plot due to a note in the dressing room for "Maria") enough time to halt his plan. Unfortunately, they also ended up interrupting the play by knocking out two key actors in the opera production (Draco and his rival suitor for Maria.), resulting in Locke and the returners improvising, with Locke trying to do his best (even if abysmal) attempt at mentioning that he'll take Celes'snote The opera's character is named Maria, but Locke, horrible actor that he is, uses the actress's name instead hand and not Draco or Ralse, and Ultros challenging Locke to a duel.
Before the production of Kingdom Hearts: Birth by Sleep, Sora's Japanese seiyuu jokingly whined about having to voice The Hero all the time and asked if Nomura couldn't give him a few more "evil" lines. And so, Nomura created Vanitas.
The famous Post 217 in a NeoGAF thread about Scribblenauts became popular enough for official artwork◊ to be made of it. Soon after, the developers confirmed that "Post 217" was an object in the game's dictionary (it blows the map up).
Prior to patch 1.8, if you went an extremely-far distance in your Minecraft world (12,550,820 in-game meters, or roughly 820 hours of straight walking) you would run across a place called the Far Lands, where the terrain would suddenly become severely distorted and laggy due to a glitch in the way in-game worlds were generated. Originally, Notch said he liked the idea of a mysterious place so instead of fixing it, he kept it in. But in patch 1.8, he accidentally fixed whatever it was in the world generation process that created the trademark appearance of Far Lands. Currently, the world simply presents you with an invisible barrier, and Notch has not yet given any word on whether or not the Far Lands will return.
Creepers are the result of a failed pig model.
Warcraft 3: The Frozen Throne's Pandaren were originally an April Fool's joke that people responded to, so they were thrown into the game, with a side mission and a hero in the orc campaign. Blizzard even gave their race a tidy backstory and are a playable race in World of Warcraft: Mist of Pandaria.
In Quest for Glory IV, every evening in the Inn there are three villagers who convene. The voice actors had great fun ad-libbing lines for them, including a rant on how one of them used to be an elephant herder (the game is set in a country based rural medieval Romania, BTW, filled with forests and swamps) but the elephants all started dying out. The ad-libbing is quite apparent as on many occasions their speech do not even match what's written in the dialogue boxes. At one point the characters even speak out of order of what's being seen at the screen. The story passed around is that the diskette version (which had no voice) was finished before the CD edition, which had voice acting. The developer loved the ad libbing that the voice actors did so much that they were not only allowed to continue but were used in place of the conversations in the diskette version, which is why they don't match.
In Grand Theft Auto III the pedestrians have a strange habit of jumping in front of the vehicle you're driving - this was originally a glitch in the AI that the developers found quite funny.
In a similar vein, in San Andreas on occasion planes would crash for no reason, a result of a less-than-airtight approach to random generation of flight paths that allowed the possibility of planes' trajectories intersecting with the ground. It was kept because it added to the hilariously crapsack nature of the game. (Though it did tend to annoy the occasional player who suddenly found their character wasted because of being in the path of said plane crash!)
Grand Theft Auto V had its key actors perform many cutscenes with motion capture suits on and delivering dialogue live (leading to the realism of the CGI). In one sequence, the actor playing Trevor was supposed to hop over a low fence, but he goofed and fell over instead, causing the actor playing Franklin to break into laughter. Rather can cutting the scene, the actor playing Trevor proceeded to deliver an epic (and somewhat scary) angry rant, in character, that was considered so good (along with Franklin's actor wisely also staying in character) that it was incorporated into the cutscene as it appeared in the game. As a bonus, Trevor's ad-libbed rant actually worked towards the continuity of the game because at this point in the game, Trevor had just left his friend Michael to, he thinks, be killed by Chinese mobsters, and his rant ties into the guilt the character is feeling about this. The scene ended up being one of the funniest in the game. Watch it here.
Many of Nathan Drake's one-liners in the first Uncharted were the result of Nolan North's improvisation while watching gameplay footage of his character.
Perhaps one of the more noticeable adlibs is on the Train level, when being chased by an attack helicopter and some more grunts show up:
Nate: CAN'T YOU ASSHOLES SEE THE HELICOPTER?! I HAVE ENOUGH TROUBLE ALREADY!!
According to North, Charlie Cutter's claustrophobia in the third game was ad-libbed by his voice actor Graham McTavish, who thought it'd be funny as Charlie freaks out while making his way between a very narrow passage.
The Onyxia encounter in World of Warcraft was intended to have her Deep Breath ability used as a finisher on groups that had a significant number of players already dead. Due to a programming bug, it was used much more frequently, and was only left in because it didn't stop people from killing her.
Most of the swearing in Brütal Legend (especially on the part of Ozzy Osbourne's character) wasn't in the original script. But Tim Schafer decided to keep it because, well, it fits in.
Apparently when Fawful's dialogue was being translated for the English release of Mario & Luigi: Superstar Saga, several translation errors were noticed but kept in the game anyway since Nintendo of America figured it would still be in character for someone like Fawful to say.
The 2009 Bionic Commando game features an Easter Egg where you can listen to one of Super Joe's flubbed lines: If you die on a certain boss in the game and try again, instead of telling Nathan to fight the boss, he tells him "You'll just have to fuck it".
Prior to the release of Red & Green, one of the developers realized that, once he took the developer's tools out, there was room for one more mon. Since there were in-game references to the Pokémon Mew (where they got the material for Mewtwo), he decided to make it playable, but without a way to get it. No information was given about it, because except for the higher ups at Game Freak, no one knew you could get Mew. He figured they could give it away later, and a tradition was born. The Mew glitch, discovered in 2005, allows you to get it normally by fooling the game into making it appear. This is due to the way the game handles numbers as code.
Pokémon Gold and Silver, in development, filled up the entire cartridge before it was even half-finished - until Satoru Iwata (now CEO of Nintendo) compressed the game so much and so well that, even with the main game finished, there was now room to also fit the entirety of Kanto, the previous games' region, as a second badge quest after beating the "normal" game.
In Episode One, as Dog is about to throw a car containing you and Alyx across a pit to the destroyed Citadel, the following exchange takes place:
Alyx Vance: Well, Gordon... unless you have a better suggestion... He is a robot. He's done the math. [to Dog] You, uh, did do the math, right? [Dog sheepishly shakes his head]
The head shaking originally was just a random Idle Animation, but during a playtest it managed to sync up perfectly with the dialogue, and the playtester thought it was both intentional and hilarious; the devs agreed and made it an intentional part of the scene.
Interestingly, the gunships firing at the RPGs as opposed to the player was originally a glitch, but they realized it added an extra challenge to the gunships and left it in (the glitch was caused by telling the gunship to attack the "most dangerous target", which the AI proceeded to assume was the rocket that could actually damage it, rather than the guy that had fired it).
A similar case during playtesting. At one point in Episode 2, you're gently prompted into throwing a grenade into a dumpster full of boxes. Do this the first time and the Fast Zombie hiding in there will toss it back out. It was originally a physics glitch caused by the boxes that would fly out when the Fast Zombie emerged, but like Dog's Indy Ploy, it was well received and left in.
Yet another random mistake ended up becoming a gameplay element: the Combine helicopter's infamous "mine spam" attack was originally inspired by one of the programmers accidentally making the helicopter shoot mines instead of bullets out of its machine gun, which slowed the game to a crawl; for the release they had a toned-down version of this as a Desperation Attack.
Samus Aran from Metroid was originally going to be a man, and is even referred to as such in the original instruction manual. One of the developers thought it would be funny if the player found out she was a woman at the game's end. Somebody agreed, and the rest is history.
In Sam & Max: Freelance Police: The Devil's Playhouse, one part in the script asked for some characters controlling the body of a giant Kaiju monster to make it do an "anime pose", intended as an homage to Voltron-type anime. The animator in charge of making the pose innocently asked the writer, "You mean like Power Rangers or like Sailor Moon?", to which the eavesdropping person in the neighboring cubicle immediately popped his head over the side and said "Oh my god, make him do 'Sailor Moon'". As a result, instead of doing a Voltron pose, the monster does a version of Sailor Moon's Transformation Sequence, which is a total non-sequitur but really, really funny.
The multi-coin blocks from Super Mario Bros. 1 started off as a glitch for what were intended to be single-coin blocks. They were fixed as development went on, but they were apparently so popular with the programmers that they were put back in as an actual feature.
Phoenix Wright: Ace Attorney had this quite a bit in terms of the text. After the game's final script was made for the English version of the game, the port team had a reading of the script. During one of these readings of the Recipe for Turnabout script they got to the part in Blue Corps and they were reading the dialog for when someone presents irrelevant evidence. The original scene was just meant to be Lisa Basil explaining that Blue Corps uses a "Super Administrative Restricted Desktop Access Password-protection system" before Maya said "That's madness!". When the staff read this part out, one of the writers jokingly shouted out: "No Maya. THIS. IS. SPARDA!". This got a massive laugh from the team and the script was edited to included the improvised joke.
During the development of Bioshock, the developers were surprised to find that the telekinesis power allowed the player to move the anchor points of trip wires as if they were regular objects. The devs then decided to tweak that behavior and have it be an actual feature of the power.
Elzam's Leitmotif always being played when he's in combat was initially a programming bug (as he was a boss that switched to your side, and boss themes have higher priority than player themes; the bug was that when he switches sides, player theme priority is added onto his boss priority rather than replacing it). Since then, it seems both fans and developers loved it, making nothing able to override Trombe!, save for music actually playing in-universe.
Bulletstorm has a saturation of swear words (including modified swear words that sound like they came out of a maternity ward for schizophrenics) because the swearing was in English and the developers of the game were Polish, so they added in what they thought were regular muttering swears and got the unholy result. Seeing as how one half of the game is about making insulting, violent kills, this plethora of insulting, violent swearing ends up as the other half of the game.
The gauges screen in B-17 Bomber was originally meant to be a debugging tool to be removed in the final product but ultimately was left in because developers liked it so much.
The iconic Miko heroine was originally a knockoff of Pocky and Rocky, but ZUN admitted in the manual for Embodiment of Scarlet Devil that he hadn't intended her to stay that way — the popularity of Touhou as "the shrine maiden shooter series" convinced him otherwise.
It happens, most likely completely unintentionally, in one of Shadow's missions in Sonic the Hedgehog (2006). The actor playing the G.U.N. official reading the line accidentally says "Head to the Wave Ocean" instead of just "Head to Wave Ocean" as was correct. You then hear one of the writers mumble "Head to Wave Ocean", correcting him, then the voice actor reads the entire line over again. This is not subtitled and appears to be just one of the many errors that weren't fixed in the game due to it being Christmas Rushed.
Sonic the Fighters exists because of a bored programmer at Sega AM2 snuck Sonic and Tails into Fighting Vipers as playable characters (the final version doesn't have them). Yu Suzuki found the gag amusing, showed it to Yuji Naka who also loved it, and a collaboration was born. Incidentally, Sonic the Fighters runs on a modified version of the Fighting Vipers engine.
16 years later, Sega AM2 did exactly the reverse. For the HD re-release, they put the finishing touches on previously scrapped character Honey the Cat (who herself is based on a Fighting Vipers character), and threw her in as a hidden fighter.
In Borderlands 2, Tiny Tina features a lazy eye that supposedly was a result of an animation glitch that the developers decided to keep. It definitely helps sell the maniacal Creepy Child vibe.
According to Word of God, Handsome Jack's lines about his diamond horse Butt Stallion was wholly improvised by his voice actor Dameon Clarke. Lead Writer Anthony Burch jokes that he's somewhat jealous that Jack's most famous line was something he did not write.
Axton was initially programmed to occasionally flirt with Maya when reviving her ("Wow. Do you work out, or..."), but the voice clip ended up triggering regardless of which character he was reviving. According to Anthony Burch, the writers took this glitch and ran with it in the DLCs, making Axton canonically bisexual.
Simon Viklund, music composer for PAYDAY: The Heist, has several pieces of party music that he composed during his spare time. One of the levels in the game has a party going on and the game needed music for the event during the heist. Simon saw he had leftover music lying around and threw them in.
The mouselook feature in Duke Nukem 3D was originally just something you could do in the map editor to help in building them. The developers thought that it would make for a useful gameplay feature as well, and decided to implement it thus.
The intro sequence for the Final Boss of zOMG! included a random player character scripted shouting "Look out! It's huge!" Enough players actually responded with "That's What She Said" that the developers eventually added it as another scripted line.
The infamous "bucket trick" in The Elder Scrolls V: Skyrim, in which you can rob people without consequence so long as you put a bucket or pot on their head. The developers discovered this trick a few days before the game was released, but decided not to patch it out because they thought it was funny. The developers have gone on record saying that they'll intentionally leave bugs in the game as long as they don't break the game or make it less fun to play (such as the bug that caused chickens to report crimes they witness the player committing, which, while incredibly funny, would not be fun to deal with for players who aren't aware of it).
Nier has "boar drifting." You can ride around some of the giant (giant) boars, and a bug made it possible to drift at high speeds like a car. When it was discovered the makers decided it was hilarious and left it in. Many players agreed with the decision.