The Missingno. (short for Missing Number) is a Video Game item or character whose very existence is actually just a flaw in the game's internal programming. Can be considered a Good Bad Bug in and of itself, but all bets are off.
May be found in a Minus World, and often exists for the same sort of reason — the game accidentally tries to load character data from a section of memory that is intended for some other purpose entirely, leading to the resulting "character" appearing. If a Missingno is noticeably more powerful than anything obtainable at the point in the game that it can be gotten, using it treads into Not the Intended Use territory.
Note that because this character was never intended to actually exist, merely encountering The Missingno. can trigger an unpredictable bevy of side effects, ranging from other Good Bad Bugs to game crashes, or even Game Breaking Bugs. In the rare worst-case scenario, The Missingno. can even corrupt the player's save file, forcing them to erase it and start the game over from the beginning. Some Missingno.s can eventually become an Ascended Glitch, though.
It's worth noting that, despite common belief, Missingno. and M-Block are actually relatively harmless; it only causes minor graphical glitches and corrupts the Hall of Fame data (and the Hall of Fame entries aren't required for playing the game anyway). There are much more dangerous glitch Pokémon, but they require significantly more effort to make appear.
Basically, a good rule of thumb to follow, is that if a Glitch Pokemon requires more effort to see or catch, it'll cause more damage to your game.
'M and Missingno. are probably the best-known examples in the series because you can encounter them without modifying the game in any way. With use of a Game Shark and a Walk Through Walls cheat enabled, however, it's also possible to run into a number of different glitched Pokémon and trainers, many of which can do serious damage to your save file if you're not careful (or just plain unlucky). This LP of Blue version details many of them and their effects.
It's interesting to note from third gen onward, "Missingno." (in gen three, "???????", "? ? ? ? ?" or variants) is intentionally put in to catch these errors.
Sonic the Hedgehog 2 had a glitch where using debug mode to create a metric buttload of a certain object would cause Sonic's palette to be overwritten, turning him green, black, and one other color (which would often also be green or black). This miscolored Sonic, nicknamed "Ashura", is a surprisingly popular "character", making bizarrely frequent appearances in fan works.
It's also been confirmed that "Ashura"'s color palette was the inspiration for Scourge's.
There's also a white recolor of Knuckles in Knuckles Chaotix, nicknamed White Echidna (shortened to Wechnia), who's actually what's left when you take Tails out of the game altogether.
Also, if you turn on Debug Mode, use Stage Select in Sonic 3 & Knuckles, go to the Doomsday Zone Act 1 as Knuckles and beat the level, the ending sequence will feature a palette swap of Sonic, Knuckles, and the Tornado.
Generally trying to get either Tails or Knuckles into The Doomsday through this method will make their sprites garbled up, as the game still recognizes both of them as being Super/Hyper Sonic, who is supposed to be the only character allowed to face this level and the game's firstTrue Final Boss.
Since the game dynamically loads enemy sprites and tiles as you go through a level, if Sonic goes too fast while in Super Mode, he can outrun the sprite loading and cause enemies or backgrounds to be garbled.
The original Mortal Kombat, while it did not have a character named Ermac (it was an April Fool's joke), did give us Glitch Reptile and Silver Goro, two glitch characters that trigger during the game's three Endurance Matches in one-player mode. Glitch Reptile occurs when you do the trick to get to Reptile during one of the Endurance Matches; you'll fight two Reptiles as a result, but the second Reptile will be a graphically glitched version of random characters in the game. You get Silver Goro when you perform a fatality on the second opponent of the last Endurance Match with Raiden or Sub-Zero, then throw your projectile at Goro as he falls from the top of the screen. Do it right, and his colors will be glitched to a weird silver for the remainder of the round. Good memories, good memories...
In the Amiga port of the second game, do the friendship on the morphed Shang Tsung, and you're now just controlling the lost brother of Glitch Reptile.
Street Fighter Alpha Anthology's hidden bonus game Hyper Street Fighter Alpha brings us hidden versions of Alpha 2 M. Bison and Sagat with scrambled Missingno. sprites. They both share a partial Evil Ryu moveset, have no super meter, and cannot be hit while standing still.
In Ultima VI, one could grab any dead body from the enemies you kill, and have a Healer resurrect it as if it were a party member. The result is a bizarre graphic mishmash (usually depicted as a pile of floating gold coins) with an unintelligible name, now a member of your party. Stats were extremely erratic, but it was good to send on a suicide mission as a distraction.
A couple of these can be seen in the Dummied Out "lost levels" of Super Mario Bros. 3, such as a Koopa with Chain Chomp sprites, as well as in certain ROM hacks.
With the help of Game Genie codes, you can also create strange power-suits. One of the weirdest turns the player character into a conglomeration of blocks that can swim in the air (a Lakitu Cloud on the map screen).
In Advance Wars Dual Strike, it is possible in the CO select menu screens to choose only one CO and leave the second slot blank, then swap the CO with itself, leaving the first CO slot blank and the second CO slot occupied. Starting the game, you are given control of a "null" CO with glitched graphics and Andy's theme song. Your CO power charges instantly and activating it will freeze the game. Fixed for the European version.
Contra's Energy Zone on the NES: The pallette for the level changes at the end to make room for that level's boss sprite. If one of the soldier Mooks follows you there, it turns into a sprite barf when the pallette changes. The game luckily keeps going as usual, and you can shoot it.
Halo 2's "Honor Guard Councillor" (Honor Guard armor with Ultra palette and Councillor or other random helmet) was supposed to be a Zealot Elite, but is glitched up due to a programming error; it was flagged to have a "dogmatic" personality, but the level contains no data for Elites of that type.
Halo: Reach had the Armor hologram glitch. If you use hologram and either go into monitor mode or die, the Hologram model will be randomized, sometimes with impossible armor combos.
In another armor ability glitch, if the player goes into Armor Lock, switches to monitor mode, and choose to delete all of the Armor Lock items from the map, the player will have permanent Armor Lock upon returning to player mode, and it will not go away until the player dies.
Some enemies from the infamously glitchy Action 52 games seem to fall into this category. For example, the sprites in Level 5 of Ninja Assault are all scrambled, but appear to be birds and a rhino-type boss. Also, most of the enemies in Spread Fire. In Thrusters, your ship turns into one of these when you crash in the second level, which is unwinnable anyways unless you have the right ROM and emulator.
There was a custom physics model for Marathon that turned the player's weapons and the BOBs into stuff like this.
Glitches (or cheating) could add a "pirated Copland beta" or "copy of Windows NT" to your inventory. Per Word of God, these were placeholder strings for a Dummied Out weapon and its ammo. In Missingno. form, they were ammo for the (normally disposable) Enforcer gun.
Tales of Destiny had such a hidden character who was Dummied Out, but still gettable because of a bug. This led to many a Misblamed as some fans believed he/she was edited out of the US version only, when in fact you couldn't get him/her in the Japanese version either. They merely corrected the bug in the US version.
One glitch in Zelda II: The Adventure of Link lets you travel to earlier dungeons, only using the graphics from later dungeons, enabling you to do things like see unused window graphics if you go from a dungeon that had no windows to one that did. However, this also results in the boss of the dungeon you're visiting being pieced together from the sprite of the boss of the dungeon you were in before, as can be seen in this video, just under six minutes into the video.
Kingdom of Loathing parodies this with the Bugged Bugbear monster, and later the Baby Bugged Bugbear familiar based on it. Neither is actually glitched, but both have chunks of their images replaced with ones and zeroes, and both spit out MySQL error messages with almost everything they do.
The Final Fantasy IGame Genie code VYUOKITE allows you to select from 244 of these at the beginning of the game. A large number of those are automatically killed when used, but a surprising number are actually usable.
As shown in the AVGN's Game Glitches episode, the characters in the PS2 Rocky game can glitch up and become this.
The dungeon crawler game Mordor has one of these called the GOBLIE. This is a very weak companion monster that would be completely useless even if they didn't always turn up dead. It arises from a flaw in the acid spit attack that some monsters have. Occasionally an acid spitter will attempt to spit on your companions, but if your character has never had a companion then it generates a GOBLIE and immediately kills it.
A common glitch in a number of Bethesda games, including Fallout 3, dead-on-arrival characters may accidentally be spawned as "living" people with no dialogue, usually eerily standing silent near puddles of blood and piles of gore.
The fourth chapter of the Epic Battle Fantasy series has "The Glitch" as a Bonus Boss. It is portrayed as a mix of static and pieces of game sprites, is immune to everything except non-elemental attacks, and has an "overflow" attack which deals zero damage, but causes instant death all the time.
In Soldier of Fortune II, enemies that have sustained ordinarily fatal damage may fail to actually die, and continue to wander or follow the player in a "living dead" state.
In Lost Isle, there's a Gleeok in Bhalstok Castle, but, due to a palette incompatibility, it displays as an all-black "Shadow Gleeok." Seriously, this guy is almost as cool as Ashura...
Dennou Coil has Illegals, sentient viruses that feed on metabugs, and may or may not have been cyberpets once.
The Nulls are an especially disturbing example, as even though the original ones were just 'Null Carriers', intended to temporarily separate cyberbodies from the actual persons for experimental purposes, it's implied that at least some of the "feral" Nulls were once people who have been separated from their real bodies, and undergone severe data corruption and decay.
.hack//SIGN and the first set of games had "Data Bugs." Glitched monsters (and corresponding wonky graphics) within the Virtual Reality MMORPG with infinite Hit Points who can send players into comas (not as permanently as the Phases). Only a game reprogramming device like the Twilight Bracelet can defeat them.
Inception has stray projections that get in the way of dreamsharing. There's Mal, a violent, nigh-unstoppable projection born from Dom's guilt, who takes form of his dead wife. She rages through the dreamscape, generally fucking everything up for the crew. There's also Dom's kids, who are less violent, but always faceless and whose appearance usually indicate danger.
In Wreck-It Ralph, Vanellope, who's a video game character, is this within her own game, Sugar Rush. She's bullied and considered an outcast by the other characters because of it, can't take part in races without the title character's help, and flickers in and out. However, it turns out she was always meant to be in the game. King Candy, actually the character Turbo, modified the code to Sugar Rush to make everyone forget, and tried to outright delete her, rendering her as a "glitch." In the end, once she claims her rightful status as a playable character, she becomes a Good Bad Bug since her glitching lets her teleport over short distances. Ralph mentions that the players love it.
Homestuck's Lord English is implied to be this, being the result of two players who share a single body entering a SBURB session that was designated for one player only, something that was explicitly stated to be impossible. Appropriately enough, he's even caused the game to glitch, causing spans of Missing Time for the protagonists.
The SCP Foundation: SCP-951, an anomalous (but apparently friendly) video game glitch named "LUCAS" AKA LOGICAL UNIFIED CENTRAL ANOMALY SYSTEM. It was deliberately engineered but flawed as it would "die" when the game ended and couldn't remember anything when the game restarted. Missingno gets a shout-out in a chatlog about the mysterious glitch:
Lizalfos ██ ███ ████, 9:40 Hey, has anyone played Pauper Rise of the Monster King? I found it at a garage sale recently and so far it's pretty cool. Anyway, the reason I bring it up is because I found a pretty awesome glitch monster that kind of breaks the game and I was wondering if anyone else found it.
ReGGie ██ ███ ████, 9:42 @Lizalfos, are you talking about lucas? lucas is fucking bro tier
Wetualo ██ ███ ████, 9:45 LUCAS is definitey up there with shit like Missingno, although I didn't care much for the way he fucked with my screen.