Damage Discrimination

"Why don't Robot Dogs fall in the pits? It's not really fair."

- Duck Guardian One from Homestar Runner

In videogames, things that do damage to one thing don't always do damage to something else. The damage usually discriminates based on what team you're on. Unfair? Well, everything IS trying to kill you.

NPC Invincibility - Non-player characters are usually completely immune to all damage, especially the inhabitants of an Adventure Town. Slash them with your sword, and they might duck in fear or yell at you, but that's about it.

No Infighting - Monsters don't usually damage one another with their attacks. Monster projectiles will explode on contact with other monsters without doing damage, or pass right through them. Monsters usually aren't affected by the hazards of their natural habitat, either.

No Team Damage - In a lot of games, Player 1 can't hurt Player 2, and vice versa if they're playing cooperatively. Sometimes there is an option to toggle this behavior on and off. Often, teammates are still susceptible to Splash Damage.

In many, many platform games, enemies will cheerfully walk through anything and everything that would kill you in one hit. This includes "friendly fire" passing through people.

Often but not necessarily a subtrope of The Computer Is a Cheating Bastard. Set a Mook to Kill a Mook is an aversion of Type 2.

Action Adventure Games

 * In The Legend of Zelda the Wind Waker, most enemies are capable of harming each other. The Moblins especially come to mind, with their wide 180° spear swing that knocks EVERYONE off their feet.
 * Not to mention Darknuts. A group of them cease to be much of an issue after circling around them, and coaxing them into attacking one another while stealing items using the grappling hook.
 * Put back in full force for The Legend of Zelda Twilight Princess. The worst example is the trio of Darknuts at the bottom of the Cave of Ordeals, who clump together into an unassailable wad in spite of their tendency to use wide ground-clearing slashes.
 * In the stealthy end section of Metroid Zero Mission where you play as Zero Suit Samus, you can trick Space Pirates into killing each other with their lasers.
 * In Metroid Prime, you can actually get metroids to attack Space pirates in Phendrana Drifts if you break open their containers with missiles from a distance.
 * Nearly all the bosses in Batman: Arkham Asylum happen to bring along mooks but they don't pay much attention to their well being. And for some, you can intentionaly cause them to flail around wildly and attack their allies.
 * The final arc of Uncharted features you vs. the bad guys vs. . Commence Melee a Trois as two out of three sides Try Not to Die.
 * Averted in Prince of Persia: The Sands of Time, near the end of the game. You're facing about 30 enemies, each twice your size, but, because you just got your Infinity+1 Sword, they are ironically too easy to kill, as they are obliterated before they can fall down, so you can't collect sands from them. This is made even worse by the fact that that bitch Farah . Luckily, they have massive swords and are inconsiderate towards any ally standing near you. You can only get more sand by collecting it from enemies knocked down by other enemies swords.
 * Averted in Time Commando, though it's difficult to achieve and usually more trouble than its worth.

Action Games

 * In Devil May Cry 3 and again Devil May Cry 4 enemies are just as likely to harm each other as well as the player, and in fact the games usually have a type of demon spawn just for the purpose of indiscriminately causing damage, such as the Wraths in 3 or the Blitz in 4. Surroundings that can also harm the player are just as capable in harming monsters as well.
 * This goes back to the first game, in fact: one bonus mission requires that you trick one enemy into killing its partner by manuevering it so that it attacks the other enemy by mistake.

Beat Em Ups

 * In Streets of Rage the explosives thrown around will hurt the enemies or blow them off their bikes even if you don't pick the enemies up and throw them, thrown knives will hurt the enemy even when thrown by their allies, and enemies who accidentally hit their allies generally get hurt. Also, Mr X has a Uzi in SOR2, and he'll mow down his own henchmen if they get in the way. It makes the fight against him a little easier.
 * One of the features that made Battletoads so infamous is that players can hurt each other in co-op mode. And that is more likely than you think. Add to that the fact that during descent levels hitting your partner with a special move (that is the only one that can obliterate enemies standing on the sides) causes them to be knocked off the rope and that when a player loses all his lives both players have to restart the level... oops!

First-Person Shooter

 * Doom was an unusual aversion of the trope in that the monsters would gladly fight with each other, though the game was carefully balanced so that exploiting this fact would take some skill. For example, the fireball-throwing imps could neither hurt each other with fireballs nor piss each other off that way--but it was possible, if unlikely, for them to strike each other with their claw attacks. "Crossfire" between different kinds of monsters almost always resulted in the monsters fighting--an imp hitting a demon with a fireball, for example, would soon have a large pink horned shaved gorilla-thing in its face and trying to chew it off. Even the novels made use of this, and one of the characters (the more well-read of the two main characters) called it the Iago tactic. Granted, the enemies are basically the embodiment of murder, death and pain, so they would fight each other.
 * Also works in Quake.
 * In earlier versions, if a monster accidentally damages itself with the splash damage from an exploding barrel, they'll try to fight themselves, ripping themselves to shreds if they can perform melee attacks or just going berserk if they can't. It was a bug though, and was later fixed.
 * This can lead to some crazy.
 * The Turok games functioned the same way, with monsters fighting each other if you could trick their attacks into hitting a monster of another type. It worked great when there was a mixture of close range dinosaurs and long range humans, since as soon as some human bullets hit the raptor, both ceased being your problem for a little while.
 * This was even a huge selling point of the most recent one, with various ways to make dinosaurs go after humans.
 * Marathon was another game where the Pfhor enemies would easily "berserk" each other even with their ranged attacks - in fact, the Fists Only runs would often rely on this strategy; gather the enemies into a swarm, move in and punch a couple of time, then back off while the overzealous enemies tear themselves to bits.
 * Also, due to the story of the game (the Pfhor are slavers who brutally rule other races), there were many cases in the series of enemies such as Flick'ta attacking the Pfhor without you even having to cause it.
 * In BioShock (series), the Big Daddies will go after anyone who attacks them or the Little Sisters they are protecting.
 * Not to mention the wide variety of Plasmids that allow you to turn Rapture into your own personal playground. Get that camera on your side from long range so the security system sends a drone after bad guys. Enrage a splicer into attacking a Big Daddy for you. And my personal favorite: use the Big Daddy Bodyguard plasmid to trick a Big Daddy into thinking you're a Little Sister. Suddenly, anyone that attacks you gets massacred.
 * In Borderlands, Raiders will attack any monsters and vice versa. They don't even wait for you to instigate. You can also kill yourself with explosions you instigated with rockets, grenades or shooting barrels. On the other hand, non-combat NPCs are incapable of being hit by attacks (shoot at them, it just hits the environment behind them.) and you are incapable of harming your allies (Aside from exploding barrels with your shots, that will still harm your allies, I think.)
 * In Unreal, it was possible to have different monster types get into combat by positioning yourself so they ended up attacking each other.
 * Left 4 Dead doesn't have friendly fire disabled for the infected, so special infected can wind up slashing each other if they get too close to each other. However, their special attacks such as the Boomer's vomit or the Smoker's tongue grab won't affect the zombies. Tanks are extremely dangerous for infected friendly fire since their punch or rock throw can kill a zombie in one hit and can cause the same hilarious result if they punch a car or forklift towards their allies. The Boomer's explosion can actually stumble infected if they are too close, but they suffer no damage.
 * The AI in Thief are capable of harming each other, though it's very tricky to get them to do so. A bowman accidentally hitting a swordsman won't cause the two to become hostile to each other. Though there are also several different "teams" than an AI can be set as (Ally, Monster, badguy-1, 2, etc) which can dictate how an AI will react to the player or other AI. For instance, a human will always attack a Zombie or Giant Spider on sight.

Hack and Slash

 * The Demons in Gauntlet (1985 video game) can harm non-demon enemies with their fireballs... but unlike Doom, this won't lead to the other enemies attacking them.
 * Magicka does not discriminate at all, especially since Magic A Is Magic A. The players and the enemies can just as easily damage themselves as each other, which is part of the hilarity.

MMORPGs

 * Averted (thankfully!) in Dungeons and Dragons Online where luring enemies into traps is a very effective strategy for non-tanking types. (And trap damage also scales for them with difficulty setting)

Platformers

 * Just about all of Insomniac's games (The PS1 Spyro games, Ratchet and Clank, Resistance, etc.) avert the "no infighting" rather spectacularly. In fact, they make it the main point of some things: In challenges where you must conserve your ammo in Ratchet and Clank, it's a very good idea to lure the enemies into killing each other by accident instead of wasting your ammo; there's also a weapon in the third installment that specifically makes enemies attack each other.
 * In Wario Land The Shake Dimension, every single thing in the environment that can hurt Wario will instantly kill any enemy that touches it. To add to this, in some stages, there are actually missions saying to use one of these obstacles to kill a certain amount of enemies. And even more so, some things only hurt the bad guys, such as being crushed flat between a column and the floor/ceiling (Wario only gets somewhat flattened without taking any damage) and water.
 * Usually played straight in the Super Mario Bros.. series, with the exception of enemies that can launch themselves spinning at you (Buzzy Beetles on ceilings, for example). Often, they'll end up taking other enemies out, whether or not you "repurposed" them.
 * Freeware game Iji avoids this as well -- the two alien species actually prefer killing each other to attacking the player. They are, however, immune to friendly fire (but not splash damage) - unless their Humongous Mecha go crazy and start firing on their allies.
 * Dark Titans in Sonic Unleashed, unlike just about everything else in the game, can and will barrel through anything in sight to get to you, including allies. This is the only good thing about them. Some bosses have similar tactics.

Puzzle Games

 * Braid mostly avoids the "no infighting" rule: environmental hazards do not discriminate between Tim and his enemies, and enemies can Goomba Stomp each other. They don't go out of their way to fight each other, though.
 * In Portal, you can trick the turrets into shooting each other with some effort. The designers even thought of this, since the turrets being hit will tell the other to "stop shooting!". They only damage you, however.
 * Although they will knock each other over. There is even an achievement for arranging this.

Real-Time Strategy

 * Myth: The Fallen Lords and its sequels didn't use it either. A grenade that took a wrong bounce could easily damage your own units.
 * Ditto for any launched or thrown projectile, such as spears and arrows. A skilled player, however, can trick enemies into destroying their own units.
 * In Total Annihilation your projectiles will pass harmlessly through your own units, but not your teammates' units. And your own units can still take splash damage if the weapon hits close enough.
 * Both played straight and averted in Starcraft -- attacks from some units that normally do splash damage, such as Reavers and Firebats, will not damage your own units, but damage from other sources such as your own Seige Tanks will.
 * Averted in Command and Conquer: Generals. Superweapons, splash damage and residual effects such as fire and toxins will damage your own units, and the GLA mobs can have individual people crushed by your vehicles.

Roguelike

 * In ToME it is possible to kill friendly quest-relevant NPCs with ranged attacks although you always "stop yourself" if you try to hit them in melee. The NPC will always resurrect immediately with no harm done. Rescuing a princess from monsters with blasts of area magic that kills the princess herself several times over is therefore a perfectly valid tactic. However, if you manage to get an NPC buried in rock with an earthquake, they are gone for good... until you leave the level and return.
 * Also, NPCs are not affected by traps, neither are you by your own traps (which affect monsters, of course).
 * Traps in Nethack do not discriminate. Many a corpse of a NPC Dwarf, Gnome, or Halfing can be found in pits in the Mines level, and many an adventurer has been saved by the same falling-rock trap that killed their pet.
 * Enemies won't deliberately attack one another unless you are magically "causing conflict", but they can hit one another with arrows or magic beams while aiming for you - the black dragons in particular can devastate vast swathes of enemies with their disintegration breath if you happen to be able to survive the blasts yourself. And if you are wearing a cloak of displacement enemies will think you're in an adjacent square most of the time - and may attack a monster in said square by mistake.

Role-Playing Games

 * In the Fallout games there's always a chance of the member of one party hitting the member of his own party or another party in crossfire, posing a further chance that the parties will in-fight, or previously neutral parties will join in with the bloodshed because a stray bullet hit them. As every Fallout 1 player knows, you do not stand in front of Ian whilst in combat.
 * In Fallout 3 you can get sneak attack criticals on your allies if they're distracted by an enemy, and also hit them in VATS (which you cannot cancel once initiated) when targeting enemies. Especially troublesome with Dogmeat, who frequently jumps up to head level to attack. This isn't helped by the game's interesting hitboxes.
 * In Elder Scrolls: Daggerfall, it isn't possible to kill plot-related characters because of the different types of NPCs employed. In Morrowind, you're able to kill for good every single NPC in-game, which can render the game unbeatable even through the "backdoor route" if you're unfortunate enough to kill the exact wrong two NPCs at the wrong time. In Oblivion, on the other hand, returns to TES tradition by making plot-relevant NPCs be knocked unconscious if you try to kill them, only to get up a few seconds later and wander around.
 * Oblivion averts No Infighting as monsters that don't get along will actively fight to kill each other.
 * In addition, traps meant for you may be sprung on monsters with the proper positioning and timing, particularly in the Oblivion Gates. Harrada plants will indiscriminately attack foes, firebombs on the ground can be shot with a bow as the enemy runs past them, foes may be lured through spike traps, knocked backward into lava, off a balcony, off the narrow spiral ramp going up parapets, caught in the Corpse Crusher...
 * The guards will attack people that they see attack you, and NPCs can sometimes even be caught by the guards stealing and then killed by the guards. There's a video of this on youtube somewhere.
 * In Monster Hunter, monster attacks can damage other monsters, leading to amusing situations involving a Rathalos dying from a random raptor attack. Occasionally, they'll outright fight one another, but more often than not, everything assumes the player to be the bigger threat they should attack causing them to attack you and nothing else even when they were hit by another monster or when they should be running from the dragon that just launched a fireball at them or a giant crab who keeps running over them.
 * That is rather justified since the player character poses much more of a threat to the dragon than a few velociprey or even a velocidrome.
 * In Blue Dragon, certain scenarios will spark a "Monster Fight" that makes some of the toughest monsters in the area terribly weakened-- often by another tough one, who won't attack your party until it's dealt with the target of its envy/hatred.
 * In the 3D Might and Magic games, different types of enemies would often fight with each other in the overworld and in dungeons, often to the point of completely ignoring your party unless you first provoke them.

Third-Person Shooter

 * In Gears of War 2 multiplayer, your grenades and bullets don't hurt your team-mates, but your grenades and splash damage weapons can kill you. This is a fairly standard version of "no friendly fire" for multiplayer shooters.
 * Of course some people posit that multiplayer would be improved by having friendly fire always turned on.
 * In Team Fortress 2 this is a necessity since the spy can disguise as members of the enemy team. The only real way to discover spies is by shooting/flaming suspicious teammates to see if they take damage.
 * Soldier rockets and Demomen stickies/grenades hurt/fling the launcher and enemies but not allies. For a while after launch, servers were not allowed to turn friendly fire back on. This also applies in a unique way to the Engineer: his sentry rockets AND bullets damage him (Spies can exploit this, although it's risky).
 * The original Team Fortress (and other Quake or UT engine games of the era) had a large number of "no friendly fire" modes. In some of them, you couldn't hurt friendlies at all. In some, you could damage just their armor (which could devastatingly weaken a Soldier or HWG, whose main toughness advantage came in their armor). In some, YOU would take any damage you inflicted on friendlies, or take a percentage of it. Of course, you could also turn friendly fire on.

Turn-Based Strategy

 * The spells and some of the long range techniques in Final Fantasy Tactics. Whoever's standing in the target tiles will get hit, whether they're friend or foe, and it's possible to put your own characters in the path of a charged long range attack, like Aim, and get hit in place of the enemy. Summons are the exception, never healing enemies or harming allies, but since they're actual beings coming to your aid, it makes some sense.
 * The only things in the sequels that discriminate are totema/scions and gadgeteer abilities. Even the summons will hit your characters.
 * The field damage-causing tiles in Fire Emblem can hurt enemies and allies fine. That's bad, as it means robbing you of valuable experience.

Tabletop Games

 * Dungeons and Dragons generally averts this, but not always - some spells and powers affect everything in the area, while others affect only enemies. It's also possible for both players and enemies to get creative and use attacks which their allies are immune to or even healed by (using a positive energy spell against undead being the most common example). There are also some Feats or other abilities that allow selective use of this trope, such as a fourth edition Feat to allow Dragonborn (humanoid dragons) to use their elemental breath without fear of harming allies - instead giving them a bonus to hit!
 * Similarly, 3.5e's Dragonfire Adepts have the Endure Exposure spell, which makes a creature immune to their Breath Weapons for 24 hours.

Live Action TV

 * In a non-video game example, Kamen Rider OOO has a pair of Finishing Moves that cause collateral damage that is magically repaired afterwards. His sword attack causes a Diagonal Cut to everything in front of him, but then everything except the Monster of the Week repairs. His Sagozou finisher causes an earthquake that sucks the enemy into the ground and drags them over to OOO for the finishing blow, but the ground heals afterwards.