Dead Character Walking

Through a glitch or various methods, one of game characters is supposed to die, but somehow is kept alive, possibly with bizarre effects. Applies to players where Game Over normally happens, or able to do unexpected actions like World of Warcraft's "travel by suicide". Applies to enemies or NPCs if they are not supposed to be immortal. Does not apply if when fatally hit, die after scripted period of time or script saved.

This is often considered a Good Bad Bug.

There are 3 methods by which this can be done:

Straight game mechanism: Simplest form of all. Just die, and some games allows you do unexpected things while dead.
Glitches: Some form of glitch allows player or enemy to die and stay around and move around, and in some cases still fight! Exploiting this can fall under Fake Skill.
Cheat code: First enable some form of cheat, then die. The interaction of cheat and dying code causes unexpected abilities while dead.

Examples of Dead Character Walking include:

Game mechanism

World of Warcraft: You can explore as a ghost when dead, and before you respawn. Before this bug was fixed, you could actually "travel by suicide", as in, die and walk while dead to the Spirit Healer nearest where you wanted to be. Now, you can only respawn at the spirit healer nearest to where you died, or on the site of your corpse.
- The game does forget where you died if you use a boat or a zeppelin to switch continents, allowing you to respawn at the new continent. Usually this is used to gain access to the Blood Elf or Draenei starting zones' tameable wildlife before the character is strong enough to get there normally.
- You can still travel by respawning a bit further along your path than your body actually was, but it's much more tedious.
- Occasionally, mousing over the map icon indicating your location will occasionally label you as being a corpse, even if you're alive.
In Mario Kart DS battle mode, a dead (read:out of the contest) driver can still drive around the battlefield, laying boxes, albeit invisible and intangible.
- Likewise in most of the newer Bomberman games, there's usually an option to allow defeated players to harass the living ones by riding around the edge of the stage in a hovering vehicle and throwing bombs at them. In some of the games, managing to directly kill one of the players this way could also revive you and let you resume playing normally, hopefully not getting killed in turn by the same player who you just offed.
Amulets & Armor has a great bug dealing with the "death cam", the red-tinted view of the world you see when you die. Your character is actually still alive, the game just disables the keyboard. However, A&A has mouse navigation too, which it doesn't disable, letting you explore (and even beat) levels while dead.
Literally in Mother 3; since the characters walk one after another, they are still able to walk around, but their sprite looks excessively tired.

Glitches

The original QWTF Team Fortress had a bug where a spy could feign death, and then start moving around as an apparent sliding corpse on the ground.
- And in Team Fortress 2, if your character's corpse is still there after you respawn, which tends to happen on instant respawn servers, it can produce weird effects. If your character talks, so does their corpse. C'mere, cupcake...
Final Fantasy VI has a couple of bugs that allow you to walk around with an all-dead (Or all-zombie) party. Of course, it's Game Over if you enter a fight, but hey.
- Final Fantasy VII has a similar glitch with Cloud's flashback sequence. Sephiroth's AI is supposed to revive Cloud should he fall in battle but sometimes Sephiroth will just wail on enemies instead, leaving Cloud down. This becomes extremely funny when Sephiroth separates from Cloud's party, leaving Cloud to walk around town by himself while his HP is at 0. Unlike the above example, there's no more random encounters once Sephiroth leaves due to plot reasons, so there's no risk of a Game Over.
  - Final Fantasy VII actually has a system in place in early-game that heals and revives the party after plot points and boss fights. If one does a game where Cloud is at 0 hp when you reach Cosmo canyon, this is the first time in regular gameplay when you are not healed after a boss. This leads to a party of Cloud alone dead in the menu when checked when his party splits up at Cosmo Canyon when first visited when the buggy breaks down. If Cloud was just dead by chance, this can be particularly frustrating if you walk outside to sample the enemies nearby or gain a quick level.
Ocarina of Time: By a glitch, you could walk Zombie Link in a dim world. A different method allows Zombie Link to walk around in regular lighting, but nothing can be done anyway and everything are all frozen and many features are disabled.
Mega Man 3 has an unusual glitch where the player could die by jumping into a pit with the usual death sound effect, but could then use the second controller codes to jump out and continue playing normally while being unable to be killed, with the side effect that they can't use the Mega Buster anymore since the game considers it to be out of energy when your lifebar is empty. The health recover items would make Mega Man alive again, but while collecting one meant you could fire your default weapon again, it also meant you're now able to be killed normally again.
There is an infamous glitch which crept from the infamous |Sonic the Hedgehog (2006) to the otherwise excellent Sonic Colors and Sonic Generations which makes Sonic, if he receives mortal damage while on a autoscrolling sequence, still slide with his dead body on the track. He can even be controlled sometimes with the directional pad!
InSuper Smash Brothers Brawl, by a certain glitch in stamina mode, both Wario and Bowser can become "zombies" where they can still be controlled and beat other players at 0 HP. Bowser could still win, but Wario cannot win at all after using this glitch.
There was a bug in Baldur's Gate 2 that would occasionally cause a monster not to register that it has died, rendering it undead in a manner the authors hadn't anticipated. This required either a reload or a hasty escape as the player would be chased around the area by an unkillable cloud of flying Ludicrous Gibs
Blood has 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.
Doom has such a bug, described in detail here, where a players killed in deathmatch becomes a mobile corpse which runs (okay, slides) around. Kinda creepy.
In the murder mystery text adventure Deadline, there's a bug that makes it possible for a character's death to be followed by that character running into the room and reacting to seeing a corpse (their own) on the floor.
In the original Half Life, if an NPC has scripted dialogue, but you kill them before they begin to speak, the corpse will speak anyway. The mouth moves and the audio can be heard, but otherwise he is dead. (This works at least on the security guard at the end of the Power Up sequence.)
Also in World of Warcraft, as one certain video demonstrates, there was a bug in which revived characters would keep using the dead model, leading to apparent corpses gliding on the ground.
- Some private servers have glitched programming which may lead to the corpses of mobs turning around to look at you as if still in combat, or even corpses that keep attacking you while not being attackable in return.
Can be done in the second Phantasy Star game. First, kill off everyone except Shir, then enter a shop repeatedly until she steals something and leaves the party. You'll now be able to walk around with an all dead party, though the second you get into an encounter, you'll die immediately.
- Also easily doable in Star Ocean the Second Story: just use a Mandrake on each member of your active party, which kills anyone it's used on. The game also has another, far more annoying inversion of this: often later in the game when facing powerful enemies that can easily kill, paralyze or petrify you, you'll end up getting a Game Over right after you use a healing item or spell to cure one of said statuses due to the game not bothering to check if someone's currently in process of being cured from them before declaring the battle lost due to all 4 characters being considered dead at the same time for a brief perioid of time (ie. a mage casts a status recovery spell and gets killed while it's going off). This means you'll often end up with a freshly-healed character standing there while the battle fades out and you're forced to reload your save.
The original Grand Theft Auto had an occasional glitch if you got into a vehicle just when it exploded, where you wouldn't die until you got out of the car again. Until you do, you can be a zombie cruising the streets in the wreckage of a destroyed car.
In Morrowind, there is a glitch where if you execute it correctly, the opponent, 99% of the time, will become immortal. That is, they have only one Hit Point left no matter what you do. The other 1% of the time, they will die instantly.
In Fallout: New Vegas, occasionally when you murder someone in their house and take their stuff, the corpse will accuse you of stealing. Which can scare the crap out of you when you think you're alone except for the dead body.
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.
Dead Space has a glitch that sometimes causes necromorphs to endlessly run around in circles, making them invincible until you block their path.
Occasionally in Conduit 2's online multiplayer, a killed enemy's sprite will remain in their standing pose, even though they are counted as killed. Continuing to shoot at these sprites (which other people will most certainly do if you don't) will give you a "Double Tap" medal.
In Steel Sentinels, a minor programming oversight causes mecha to not die until their hit points are negative- meaning that mechs with precisely zero hit points will cling to life with an empty health bar until they are felled by a stray point of damage. Although this glitch isn't much use against machine guns or lasers, it is immensely satisfying to complete a Fleet Capture the Flag objective while technically dead.
Minecraft mobs have a glitch where if you kill them, and exit quickly and on return they will be alive and moving around in whatever position in dying animation they was in when you exited.
Driver3 : Odd things happening during replays, where events play out incorrectly, sometimes leaving Tanner dead for half of it.
Certain versions of Touhou 7: Perfect Cherry Blossom has Merlin Prismriver continue attacking even after the Prismriver sisters are supposed to be defeated, potentially killing your character. So much for sisterly solidarity. This bug has been mercilessly ridiculed by Doujin artists.
Call of Duty has glitches with corpses, from just silently talking faces to really strange animation and postures. Cracked page explains it the best...
Fallout: New Vegas: Has a similar bug as Minecraft game. If you save while character just died, there's various bugged things that happen, from floating hollow heads to weird corpses that's still alive. Cracked explains it the best again...
A glitch in Spider-Man and Venom: Separation Anxiety occurs if you are on your last life, and you and a boss kill each other at the same time. The next level would load as normal, but you would suddenly be back to life with zero lives. Even better, since the game thinks you're already dead, you have infinite health, making the rest of the game a cakewalk.
A rarely triggering glitch in Soldier of Fortune can cause an enemy to not notice they're supposed to be dead. Due to the game's damage system, this can result in an enemy continuing to run around and shoot even after they've been reduced to just a bloodied torso.

Cheat code

Super Mario 64 has this if the cheat code for float is used, and Mario dies, as long as the death animation is not completed before float is used again.
- There's also a non-cheating method involving having your death blow knock you into a cannon. You can launch out of the cannon and won't die unless you touch the ground. If you're wearing the Wing Cap, you can even fly around as a zombie!
  - An extension of the same glitch allows you to fly around without using the cannon as well: Hurt yourself to the point where fall damage can kill you, then equip the Wing Cap. A correctly timed three-jump combo will make you fly, so if you jump once, then twice off a cliff and trigger the last jump just as you land, you will take lethal damage but still begin flying. Unfortunately the impact "flattens" Mario and thus you can't jump more than a few feet up. Touching the ground at all causes Mario to die, which will happen eventually when the Wing Cap timer runs out. Interestingly enough, once you are "dead" coins and hearts will no longer heal you, so you absolutely cannot save yourself.
In The Simpsons Hit & Run, there is a variation that lets you drive a wrecked car. Not surprisingly, It’s VERY slow.
- This doesn't even require cheating at all - but doesn't work with all vehicles. You can bring the car back to fully normal by touching a repair wrench.
Typing "give health" into the console in Quake will cause the player to assume a bizarre undead state where they're lying on the ground as a corpse, yet can still jump, look around, shoot and even kill enemies.
If using an infinite health cheat code on Conkers Bad Fur Day in multiplayer, the character will remain alive no matter how many hits they take. If, however, they take a hit that should kill them, but thanks to the cheat code, doesn't, which includes many a one-hit kill strike like a run-in with a chainsaw or a headshot by either a knife or a gun, the character will still go through their death animation as well as lose the weapon they had, but they'll be brought back to life, albeit missing bits and pieces of their body depending on how severe an attack it was. They will also still be able to take even more damage and similar normally-one-hit-kills unphased even after missing most of their body parts. The only thing that can kill a character even with the code on is when they are hit by something that'll make them explode, including a strike from a grenade launcher, a bomb, or falling an incredibly high height (also if you cheat further, a few characters can't be killed even with this, or maimed otherwise for that matter).
The early Playstation game Loaded had a cheat menu to immediately boost the player to full health. If the player had just been killed, however, it would allow them to move around the map as an invulnerable splattered corpse. Unable to shoot, open doors or pick up keys. However, they could still detonate smart bombs, whilst a second player could deal with the doors for them, but still not much point.