Friend or Foe?

""Are you friend or foe? You never can tell with some people.""

- Ramba Ral, Dynasty Warriors Gundam 2

""Neither enemy nor friend.""

- Gray Fox, Metal Gear Solid

Sometimes, in combat, it's hard to tell who your friends are.

In a combat zone, firing on, questioning, or fleeing your friends is common. With Due Respect may not be very respectful, as the junior cannot tell whether the character speaking to him actually is his senior officer.

Confusion deliberately induced by the villain is Let's You and Him Fight. Dressing as the Enemy can also lead to this, as can catching someone unawares. Unfriendly Fire often relies on this to cover up the murder. Extreme forms of Divided We Fall can lead to this. The Berserker often does not care which he's attacking. Can apply even if one character does know who is there, but has been lied to about his character, which often leads to the problems of Would Not Shoot a Good Guy.

This is often even worse before or after the actual fighting, while you are sneaking about territory that you know that the enemy is about somewhere, or into his stronghold, especially if you are a prisoner or trying to contact one. (See "Alone with Prisoner" Ploy.)

Note that the characters do not have to be in doubt—they can just be wrong. They may (briefly) believe the approaching forces to be friendly, or they may go through all sorts of fighting and fleeing believing them to be foes. Get It Over With often reveals this trope was in play.

Super-Trope of Cavalry Betrayal.

Truth in Television. In Real Life, 'friendly fire' (AKA "blue-on-blue engagement" among NATO forces, fratricide, or "own goal" as the Brits call it) has been a serious issue in many conflicts. This problem goes far enough back that it's responsible for battle cries, military uniforms, battle standards, and heraldry. Books have been written about the resulting investigations. Armies have developed many innovative designs to avoid it, but have never been 100% successful—partly because if you can always identify your friends, so can your enemies.

Not to be confused with the Game Show hosted by Kennedy.

Contrast with Friendly Fireproof, where you can hammer away at your teammates and not cause any damage at all.

Anime and Manga

 * In the Backstory of Mobile Suit Gundam 00, Sumeragi and mistook each other's forces for the enemy and effectively obliterated their allies before realizing it. Sumeragi's love interest died in that battle, too.
 * In Gundam Wing, the five Gundam pilots start out wary of each other and sometimes end up fighting among themselves after destroying their mutual enemy. None of them were told by their handlers that there would be others like them, so the Heel Face Turn they all had concerning the objective of their mission prior to launch made them into each other's potential enemies. Unlike what the fandom would have you believe, they don't come together as a five man team until near the end of the series, but they do often work in pairs of varying combinations once the misunderstandings are cleared up.
 * In Code Geass, Lelouch uses the enemy's Identification: Friend or Foe (IFF) signals to track their movements.
 * In the RS arc of Pokémon Special, Gabby and Ty are horrified when they realize their boss is Archie and has thus prevented any news of Team Aqua's crimes to go broadcast. What's more, they witness him and Maxie teaming up and deciding to go to the Cave of Origins together in a stolen submarine. Gabby immediately wants to announce the truth to the rest of the news station, but Ty points out that if their boss was the leader of Team Aqua, it may already be filled with undercover grunts and the two of them wouldn't know who they could trust. Thankfully, in a slight aversion, the two decide to ditch the news station entirely to inform the people they know they can trust: the Gym Leaders.
 * In the Mobile Suit Gundam sidestories known as MSV, there is a Mobile Suit called the GM Camouf, which was a Zeon MS designed to look like a GM to fool the Federation. However, it proved to be a disaster when a group of Camoufs were destroyed by their own Musai transports!

Comic Books

 * In The Red Sea Sharks Tintin and his friends make an escape riding camels. Mull Pasha (Tintin's old foe Dr. Müller), head of Bab El-Ehr's forces, phones his air force commander to send a squadron of Mosquitos after them, adding "armoured cars are already on their way". Due to a misunderstanding, the Mosquitos fire on and destroy the armoured cars pursuing Tintin and co.!
 * In The Avengers, after Hercules had been rendered comatose, Zeus brought the Avengers to Olympus and attacked them. A figure came to their aid, and Zeus fought him for some time, before he realized it was Hercules, risen from his sick bed to help his friends. Zeus is deeply grieved by his folly.

Film

 * In the battle royale at the end of Blazing Saddles, The Waco Kid punches Black Bart before realizing who he is. He pats his cheek by way of apology and then throws himself back into the fray.
 * Played straight in the German film Berge in Flammen ("Mountains Aflame", 1931), set in World War I in the Alps. Austrian soldier Florian Dimai, played by Luis Trenker (who also co-directed the movie and wrote the semi-autobiographical novel on which the screenplay was based) returns from a dangerous reconnaissance behind enemy lines with the news that the Italians are about to blow up a giant mine beneath the Austrian positions and is shot in error by one his own comrades who mistakes him for an approaching Italian soldier. He still delivers his message and the men leave the position in the nick of time before it is blown to smithereens, but in the epilogue set in 1931 Dimai has only one arm.

Literature
"Guard: Friend or foe? Mort: Which would you prefer? Guard: ... Pass, friend."
 * In Dan Abnett's Warhammer 40,000 novel Titanicus, Cally Samstag had the troopers flee after one of them sent a message. They are still tracked down by skitarii. When they ask for them to Get It Over With, the skitarii say they have sent for a rescue and then realizes that they thought it was an enemy.
 * In Sandy Mitchell's Ciaphas Cain novel Death or Glory, when he made vox contact with Sergeant Tayber, Tayber refused to give him his position because he didn't know he was really a commissar, and went to meet him instead; this gives Cain hope that he has hit on a competent officer. Later, Lieutenant Piers is about to open fire on their orkish vehicles, and when Cain hails them, still demands that he prove it.
 * In Lloyd Alexander's The Kestrel, a battle-mad Theo shoots a Regian soldier before he realizes it's actually his queen (and, to make it worse, his betrothed) Dressing as the Enemy.
 * In Dan Abnett's Gaunt's Ghosts novel Only In Death, in an apparently haunted stronghold, the Ghosts repeatedly bring up guns only to discover they were about to shoot their own men. No friendly fire occurs, although sometimes because another soldier stops it.
 * In Blood Pact, Criid attacks a figure in the streets; he wrestles with her until he can point out that he's Gaunt.
 * In William King's Warhammer 40,000 novel Space Wolf, when Ragnar escapes the caves, he faces a lot of guns in the hands of Space Marines. Although he thinks they recognize him, he is very, very, very careful, because it would be irony indeed to escape the Chaos Space Marines to die at the hands of friends.
 * In Grey Hunters, they find soldiers, and Ragnar spies on them to discover that they are loyalist. He is very careful about contacting them, in order to avoid provoking a fight.
 * In Lee Lightner's Sons of Fenris, when Ragnar is leading Space Wolves in the jungle, they reach the city, and find that the comms don't work, and they are taken for enemy. Ragnar has to charge through the attack to make contact. Later, when Ragnar smells that there are other Space Marines in the city, he and the rest of the Wolfblade ready for combat, just in case.
 * In Graham McNeill's Warhammer 40,000 novel Storm of Iron, the Chaos forces herd prisoners toward the Imperial outpost. They are slaughtered, and the forces learn the positions of the Imperial guns.
 * In Graham McNeill's Warhammer 40,000 Horus Heresy novel False Gods, Davin's moon is so mist bound that Loken is always bringing up his gun to shoot before he recognized an ally. More seriously, remembrancer Petronella Vivar takes it upon herself to go to the battlefield, and her shuttle goes unrecognized and is fired upon.
 * In James Swallow's The Flight of the Eisenstein, Garro refuses to shoot on a Thunderbird without hailing it to find out why.
 * In Graham McNeill's Fulgrim, Lucius
 * In Graham McNeill's Warhammer 40,000 Ultramarines novel Dead Sky, Black Sun, after a fight, take Uriel and his companions prisoner because they might be friendly—though they think probably not.
 * In The Killing Ground, when the Space Marines make contact with Imperial forces, they first scout them carefully; Pasanius assures Uriel that their machinery is well-maintained, which points toward Imperial forces, but they still meet them with some trepidation, as there is no way to be sure. Later when the Grey Knights arrive, they take Uriel and Pasanius prisoner—none too gently—because they might be tainted and so enemies.
 * In Edgar Rice Burroughs's The Gods of Mars, John Carter, trying to escape, attacks the approaching jailor—only to realize that it wasn't the jailor, it was his own son. Briefly, he even thought he had killed him.
 * In one battle in David Eddings' The Diamond Throne, a force masquerading as Pandion Knights is attacked by a force of legitimate Pandions. As both sides would be in the same armor design, the legitimate Pandions wear colored armbands to identify friend from foe.
 * In Jim Butcher's Dresden Files novel Dead Beat, makes this very difficult to figure out.
 * In JRR Tolkien's The Return of the King, the sight of the fleet causes panic in Gondor until its flag becomes clear.
 * In The Children of Hurin, Turin, having been captured by Orcs . This triggers a major Heroic BSOD.
 * In H. Beam Piper's Lord Kalvan of Otherwhen, Kalvan discusses battle cries to keep their forces from attacking each other. At the battle itself, some of their foes attack others on their side; after it, Kalvan talks with one prisoner, who indignantly declares that he had been shouted his battle cry at the top of his lungs.
 * In Wen Spencer's A Brother's Price, Jerin, escaping, threatens to shoot Cira. She barely manages to get past the gun; then she helps.
 * In Endless Blue, Mikhail immediately deploys Reds to guard on the crash; when he is asked what if someone approaching was friendly, he says they will learn that they are not. He tones down the orders shortly. An officer complains that they can not use IFF—Identification Friend or Foe—to recognize anyone, and Mikhail orders maintaining radio silence, which will keep anyone from finding them.
 * Terry Pratchett's Discworld novel, Night Watch, has a group of the old City Watch defending Sam Vimes, while another group of Watchmen (led by an escaped criminal) are trying to kill him. The friendlies happen to be passing a lilac bush, so they each break off a blossom and stick it on their helmets. This becomes a tradition in later years, with the survivors of the battle wearing the lilac every 25 May.
 * In Interesting Times the Magitek terracotta warriors springing up beneath LordHong's army causes the soldiers to flee... towards the armies of his rival Lords who only joined forces because 7 barbarians had taken over The Empire. The soldiers assume that they're being attacked by Lord Hong and start fighting. It's noted later that these fights actually killed more of the soldiers than the terracotta warriors.
 * Asked by a palace guard when Mort, channeling Death himself, tries to enter.


 * Another example, in Discworld/Jingo. While Vimes is having a talk with 71-hour Ahmed, they are attacked by a random Ankh-Morpork patrol.
 * In CS Lewis's The Silver Chair, having killed, they set out warily into her underground kingdom—her subjects are setting off firecrackers, and they fear signals—but once they capture one to question, , and matters are settled all around.
 * In James Swallow's Blood Angels novel Deus Encarmine, many Blood Angels, driven into the Black Rage, fall blindly on each other—an effect that the survivors actually admit frightened them. When they counter-attack and take down the enemy ship, the Word-Bearers' helots are driven mad by the psykers' deaths and fall on each other—blocking the Word-Bearers' way, so they slaughter them, too.
 * Part of the formula of the Hardy Boys series has the heroes, at the climax, briefly mistaking The Cavalry for enemy reinforcements.
 * In E. E. "Doc" Smith's Triplanetary, in the Atlantis section, Phryges is held at gun-point by a woman while he is undercover—and he realizes it's his childhood friend Kinnexa. She insists on his turning around so she can check for a scar to be sure it's him.
 * In Patricia C. Wrede's Mairelon the Magician, Kim is accosted as she comes out of the pub, and blacks his eye before she realizes it's Mairelon.
 * At the end of A Good Clean Fight by Derek Robinson, a (British) SAS patrol is ordered to intercept the survivors of a German bomber that landed out in the desert. They do, and capture the crew. A (also British) fighter squadron has recieved the same orders, but when they reach the bomber they see that someone's been there before them. They follow the tire tracks—and thoroughly strafe the SAS patrol and kill everyone they see. It happened.
 * In Andy Hoare's White Scars novel Hunt for Voldorius, the Raven Guard, seeing someone arrive, discuss the possibilities—not allies of the Chaos forces, since they arrived secretly, but they could be Chaos forces that are rivals, and they don't see any way they could be Imperial. It is fortunate that one White Scar scout made out some of the Raven Guard and voxed an abort to the Thunderbirds, and the Raven Guard intercepted it; there was nearly a fraticidial bloodbath.
 * In L. Jagi Lamplighter's Prospero's Daughter trilogy, at one point, someone pursuing Miranda by boat suffers a fatal accident on rocks. Only later does she learn he was one of her brother's men, trying to warn her of somthing.
 * In Robert E. Howard's "Beyond the Black River" Conan the Barbarian talks to a Pict in his own language to trick him ashore so he can kill him and steal his boat—to Bring News Back of the Pictish attack.
 * In Poul Anderson's A Midsummer Tempest, trying to reach the royalist forces brings up great fears of this being a problem; Rupert thinks he should not try to cut his hair so he can prove who he is, quickly.
 * In the Dark Tower series, Roland mentions that he and his best friend Cuthbert killed their friend Alain after mistaking him for an enemy scout.
 * In Edgar Rice Burroughs's The Monster Men, Sing shoots at ships he takes for pirates, though he knows they might not be, because being taken by pirates is too horrible to risk.
 * In Michael Flynn's In The Lion's Mouth, a secret espionage war has friendly fire accidents: one man is killed by another on his side, because of his cover. Later, Dominic Tight is targetted

Live Action TV
""So we didn't need the bloody lecture on "foe" after all, then!" "Only a friend will help you with your grammar.""
 * Another comic example is the sketch in Rutland Weekend Television where a guard asks a man in a carrot suit "Are you friendly or foeful?" The carrot man replies that a more correct challenge would be "Art thou friend or foeman-carrot?" After a lengthy discussion about the correct adverb of "foe", the carrot admits he's a friend.

"Guard: Halt! Who goes there, friend or foe? ???: Foe! Guard: Uh...oh! [shoots] Officer: [stumbles on-screen, speaking carefully] Well done, Foster...just checking. [collapses]"
 * In the finale of Battlestar Galactica,
 * Earlier in the series, Starbuck engages in a shootout with terrorists; and accidentally hits her perpetual UST partner Lee Adama. Possibly due to her using the Guns Akimbo and Leap and Fire tropes.
 * The X-Files: Agent Scully, in an effort to salvage her career, is paired with a young up-and-coming FBI agent. Who shoots a murder suspect who turns out to be holding a camera. Not only that, the bullet goes through his body and hits Scully. One assumes his career took a nosedive after that.
 * It's also a common trope for the show in general. The tagline is "Trust No 1" for a reason. It's very hard to keep track of who is friend and who is foe, especially since that changes from week to week.
 * Played for comedy and subverted in Do Not Adjust Your Set. The exchange went:


 * Happened on Meerkat Manor, when the two halves of the drought-divided Whiskers clan were about to attack the "strangers" they'd spotted in the distance, but fortunately recognized each others' clan scent before any harm was done. Justified by meerkats' poor distance vision and intense territoriality.
 * An episode of MASH (' C*A*V*E ') dealt with the 4077th's issues with a well-meaning artillery barrage. It's not the only time it happens.
 * In Deep Space 9 the crew are taking a captured Jem Hedar ship on a covert mission when they are attacked by the Federation ship USS Centaur. Fortunately there is little damage on either side.

Mythology

 * In Irish legend, when the hero Cuchulainn went into "warp-spasm," he was invincible—but could not distinguish friend from foe.

Real Life
""If British planes appear, we duck. If American planes come over, everyone ducks. And if the Luftwaffe appears, nobody ducks.""
 * "At least" would be appropriate. People playing "scenario" paintball (in the woods with ambushes and camo, as opposed to open field with team-colored shirts and short-range engagements) and airsoft realize very fast how the only thing more accurate then incoming enemy fire is incoming friendly fire indeed.
 * During the Battle of Chalons in 451 AD, after the Visigothic King Theodoric had died, the Visigoths and Romans had managed to fight Attila the Hun to a standstill into the night. In the darkness, Thorismund, the new Visigothic king, was nearly killed when he rode toward Hunnic lines thinking they were his own soldiers. Luckily for him, he realized the truth in time and booked it. This was mere hours after his father died. The Visigoths almost lost two kings in a single battle.
 * It has been estimated that during any particular war since the dawn of the gunpowder era, as much as ten percent of the causalities were the result of this trope.
 * Confederate general Stonewall Jackson was shot by some of his own men and mortally wounded when they mistook him for a Yankee in the dusk when he returned from a reconnaissance ride late during the battle of Chancellorsville (1863).
 * This was a real problem for much of The American Civil War. Confederate soldiers in particular tended to wear non-standard uniforms, due to their government generally being less organized and specifically having serious problems supplying the troops. If you didn't have an overcoat you might take one off a dead or captured Yankee. Combine that with poor communications in a pre-radio era and the huge amounts of smoke produced in combat, and the general confusion caused by combat, and Friend Or Foe happened many times.
 * During World War I, being shelled by your own side's artillery was such a big and common hazard, that in the German army there was a much-used saying: "Der schlimmste Feind der Infanterie/Das ist die eigene Artillerie" (The worst enemy of the infantry is the own artillery).
 * World War Two also had its share of incidents:
 * The United states did field tests of the practicality of camouflage uniforms. By this I mean they took a handful of infantry, fitted them with uniforms, put them on the front and see how things turned out. Well the germans were the only ones in the theater that had their infantry use camouflage uniforms in appreciable numbers. With that knowledge and the fact that its on this page it shouldn't be surprising that most the infantry invovled in the test decided to trade in their experimental uniform for the standard issue.
 * In Antony Beevor's book D-Day: The Battle for Normandy, he retells a German joke made at the expense of the USAAF (and the Luftwaffe):


 * A very awkward battle from Estonia: As it happened, the Soviet Union and Nazi Germany were both invaders. While the Soviets forcefully conscripted Estonians, the Nazis only took volunteers; either way, there were entire units of Estonians on both sides. During a particularly dark night, one of the Soviet-Estonian companies encountered a Nazi-Estonian company while marching through the forest.

Since both sides spoke Estonian, neither unit realized they were marching with the enemy—but when they did, all hell broke loose. Due to low visibility, the soldiers dropped their weapons, grabbed bayonets or knives and then held their weapon in one hand and with the other reached to touch each others' heads under the helmets. This is because the Soviet conscripts had shaven heads while the German army let the volunteers' hair be—so they determined who was friend or foe by haircut.

You could say it was the most awkward and largest Knife Fight in military history.
 * Operation Husky: 144 C-47s were on approach for a night-time landing operation in Sicily but were fired upon first by allied ships and then ground forces. A german air attack had occurred just minutes before the C-47s were arriving near the island and the naval AA gunners were simply too nervous to think first and shoot later.
 * Operation Baseplate: 900 German fighters and fighter-bombers were launched in a surprise attack to destroy allied airfields in the Low Countries. The attack had failed with 300 planes and 237 pilots lost. Many of the casualties were caused by the secretive nature of the operation which resulted in much of the German Army and Navy in not being informed of a German Air Offensive which in turn led to many German AA batteries opening fire on their own planes.
 * A number of friendly fire incidents have happened in Afghanistan during the War On Terror:
 * American-on-Canadian: The Tarnak Farm incident of April 2002. A U.S. Air National Guard F-16 mistook Canadian Army soldiers for Taliban fighters with anti-aircraft weapons. The farm had previously been used as a firing range by the Taliban, but was now being used by the Canadians for anti-tank and machine gun exercises. Although denied permission to attack by the controlling AWACS, Major Harry Schmidt believed the soldiers on the ground were targeting his flight leader, and bombed them in response. This cost the lives of four soldiers while wounding eight others.
 * British/American-on-Afghani: During April 2006, British convoy called in an airstrike from American warplanes on Afghanistan police officers, mistaking them for attacking Taliban forces, killing one and wounding twelve. Note that in this case, the U.S. pilots were attacking as specified by the British, and not making an error of judgment of their own.
 * British-on-British: In Sangin Province during August 2006, an RAF Harrier was called in to assist British troops during a firefight with the Taliban. The Harrier strafed the British instead, missing the enemy by 200 meters. This angered Major James Loden of 3 PARA, who called the RAF "Completely incompetent and utterly, utterly useless in protecting ground troops in Afghanistan." Some British paratroopers have even said they prefer American air support to the Royal Air Force.
 * American-on-British: In a case of Poor Communication Kills, one of two American F-15Es called in to support a platoon from 1 R ANGLIAN dropped a bomb on top of the unit, killing three soldiers. The forward air controller in the platoon, Sergeant Mark Perren, was not issued a noise-reduction headset; the platoon was under heavy fire by the Taliban, making it hard to hear radio transmissions. When the F-15E pilot repeated misheard target coordinates for confirmation, Sergeant Perren wound up confirming his own location as the target.
 * British-on-Danish: In Helmand Province during September 2007, British soldiers fired Javelin missiles at a unit of Royal Life Guards, killing two of them. The British soldiers had mistakenly identified the Royal Life Guards' camp as a Taliban hideout, and requested permission to attack. The Royal Life Guards, not realizing they were the ones being targeted, granted permission.
 * Dutch-on-Dutch/Afghani: During January 2008 in the Uruzgan Province, a unit of the Dutch Army attacked a composite group of Dutch and Afghan soldiers that they had mistaken for the Taliban. Two Dutch and two Afghani soldiers were killed before the firefight ended.
 * American-on-British: In January 2008, two Apaches, one British and one American, were called in to support a unit of Grenadier Guards and Afghan forces fighting the Taliban in the Helmand Province. The British Apache opened fire on the Taliban, while the American Apache attacked the Grenadiers. Only one person was wounded. After the incident, a high ranking British Army officer claimed that in contrast to the UK's full-time professional air forces, most American pilots are merely reservists, implying that this is why British-on-American friendly fire incidents never happen.
 * British-on-British: Later that year in July, a British WAH-64 Apache was called in to support a unit from 2 PARA against Taliban fighters. After attacking the Taliban positions, the Apache mistook the 2 PARA unit for more Taliban and opened fire, wounding nine of them.
 * German-on-Afghani: In early 2010, a unit of German soldiers arriving at the scene of an earlier firefight where the Taliban had ambushed a bridge-laying and mine-clearing operation encountered Afghan Army soldiers in civilian vehicles. After the Afghanis ignored an order to stop, the Germans fired on them, killing six of the soldiers.
 * American-on-Pakistani: In November 2011, a joint 150-man U.S. and Afghani unit came under fire and called in an airstrike. Miscommunication between U.S., NATO, and Pakistani forces led to two Pakistani border posts being destroyed, killing 25 Pakistani soldiers.

Radio
"Eccles: Who's dat? Halt, who goes dere? Ned Seagoon: Have no fear, I'm Q. C. Hairy Seagoon - defending council in the Nugent Dirt case. I have on me several documents of identification--including a letter of personal trust from the Commander of the British Army; a memo of recommendation from Mr. Anthony Eden, the Foreign Secretary; a special pass signed by Mr. Clement Attlee, Leader of the Opposition; and last but not least, a permit to go where I please signed by the Prime Minister the Right Honourable Sir Winston Spencer Churchill. Eccles: Friend or foe?"
 * Played for Laughs in an episode of The Goon Show in which Willium shoots at Seagoon and then asks "Friend or foe". And then there was "The Phantom Head Shaver of Brighton"


 * In The Hitch-Hiker's Guide to the Galaxy radio series, Arthur is asked this, to which he sensibly replies, "Do I know you? ...Well, without knowing you it’s hard to tell. I mean I quite like some people, others, not so much." He later goes on to explain that very few of his friends "have piercing red eyes, black armour, and laser rifles."

Tabletop Games

 * The Tactical Shooting supplement for GURPS warns us that this is a very real possibility in a chaotic and poorly lit gunfight.
 * In Warhammer 40,000, Khornate berserkers are out for blood for the blood god, skulls for the skull throne, and they don't care whose. This includes all the World-Eaters traitor Legion—among whom Kharn The Betrayer is particularly noteworthy, having attacked his own legion when they sought shelter against deathly cold rather than fight on.
 * Despite this, he's a pretty fun guy.
 * And there's also the Chaos Dreadnought who every turn has a chance to go batshit crazy and turn all of his available weaponry on the nearest target regardless of affiliation.
 * Many gamers will have a personal story of being toasted by an overeager ally with more fireball radius than he intended. Later versions of the games (especially those with magic-produced kabooms) even start adding abilities that allow true friendly fire; how about a raging infero, 40 feet across, with built-in 5-foot safe squares in a random pattern?

Video Games

 * In the first Fallout game, you had to take a separate perk to have the computer mark your friends in green. Without it, you had to remember (or, in some cases, guess) who was on your side. Or you could just "check". If the game tells you "Ian", "Tycho", "Katja" or "Dogmeat", chances are good that they are friendly.
 * So, so true in most online shooters. If you never accidentally fired at your teammate, you probably never played a shooter online.
 * Fortunately, many games give you the option of turning Friendly Fire off. Lag and collision detection glitches can also result in team kills while, say, firing on an enemy to assist with a kill if your teammate moves near your line of fire.
 * Worse in Soldner, where every player could customize his outfit and color, meaning only the teamnames above their heads showed who was on your team. Enter a bug where sometimes a teammate won't have this blue name displayed...
 * Call of Duty gets this problem as well - it's surprisingly easy to get the game to not display a teammate's name over his head even as you're staring right at him, though thankfully there's normally no friendly fire. Then you enter Hardcore mode, where friendly fire is enabled, the name-tags are disabled, there's no radar to show teammates' locations unless a UAV or spy plane is in the air, and health is reduced to the point that a single bullet with pretty much anything is an instant kill. Getting kicked from the server for too much accidental teamkilling is not at all uncommon.
 * This is particularly egregious in "real world" first-person shooters, particularly WW 2 shooters. Often, the only difference between the uniforms worn by the soldiers on your side and those worn by the soldiers on the enemy side are a slight difference in the shade of khaki. In sci-fi shooters, you at least have more flamboyant, easily distinguished uniforms, or the enemy will be outright alien and easy to spot simply by their profile.
 * The main reason to play as an Engineer in Mass Effect or Mass Effect 2 is to scramble enemy robots' IFF targeting and make them go on a berserk rampage through their own side.
 * In Fable, firing on a neutral character causes them to decide that the player is trying to kill them, and they promptly attack.
 * Fable has a huge problem with this. Trying to win any large-scale battle with non-scripted allies is virtually impossible because they end up flashing red any time the player hits them- even if it's in such a way that neither the player nor the ally has any way of knowing where the shot came from!
 * The Elder Scrolls IV: Oblivion had a similar problem, although the AI was a lot more understanding with accidental hits (with NPCs just admonishing you for being sloppy, unless you hit them quite a few times in a row).
 * It helped that they explicitly added a way to yield. Morrowind on the other hand had Escort Missions that were nigh impossible thanks to the NPC in question attacking every enemy in sight with his bare hands and often standing in your way in the process—if your aim was a hair off, they'd turn on you and force you to kill them.
 * The original Diablo game had friendly fire enabled.
 * Its quite difficult to avoid hitting fellow survivors in Left 4 Dead (well, unless you're the AIs). There's even an achievement for getting through a campaign without any friendly fire incidents.
 * Because the AI bots fire magic bullets. As of July 2009, less than 5% of the players have acquired the achievement badge for Safety First.
 * In Wing Commander Privateer, it's trivially easy to fall afoul of this with the starting radar, which doesn't give target types any color coding; the militia forces fly the same ships (with different color schemes that are hard to notice until you're right next to them) as the pirates and Retros, further compounding the problem. More advanced radar models color-code contacts, making it much easier to determine who is or isn't a legitimate target.
 * In the main games, for the most part your opponents will be flying ships entirely different from your own side's, so it's easier to tell who's who. This doesn't, however, help too much if your wingman flies right into the path of the torpedo you just launched, resulting in an insta-kill of said idiot and everyone else declaring you a traitor, in all but the very first game (where there's no penalty for killing your wingman).
 * On a related note: during a mission briefing in the original game, one wingman "Maniac" is so much trouble that the player is given permission to shoot him if he gives you too much hassle. There is also a Friend or Foe missile used throughout the series that locks onto targets if it doesn't broadcast the right friendly comm code, however the comm system can get damaged in fights.
 * Taken to an extreme in Team Fortress 2. While it is very easy to see who is friend or foe due to the brightly colored team uniforms, the Spy class has the ability to disguise as any other class and in the process take on the name of another player. As the Spy also has an instantly lethal back-stab attack and it's not always too easy to notice the spies on behavior alone, it has become common to "spy-check". That is, shoot at your own team, preferably at close range. Anyone who dies or starts running, IS A SPAH!. This is because you can not hurt your own team. The best class for spy-checking is the Pyro, as a quick puff of flame will render both the Spy's disguise and his invisibility watch useless. Pyros are in fact expected to set fire to anyone and anything in order to find spies, and a Pyro who does not do this will most likely be subject to a spy-check himself.
 * Averted in the Spy's own "Meet the Spy" video. The Soldier's point-blank shotgun blast would have been a perfectly valid tactic in game, resulting only in a BLU spy annoyed at being interrupted mid-monologue. Instead, the Spy's head explodes in a shower of Ludicrous Gibs.
 * Averted in Command & Conquer: Red Alert 2 and Red Alert 3, Allied Spies and/or Imperial Sudden Transports can disguise themselves as enemy units, but will never be mistakenly fired on by their own side. The only way for an enemy to attack them without using scouts to break their disguise is to force-attack (explicitly give orders to attack, as units without orders will not engage them).
 * During the blackout at night during a thunderstorm in Modern Warfare 2, there's a very intense scene when a lightning strike shows a group of soldiers crossing the street some 30m in front of the squad. They don't reply to the code sign by Sgt. Foley, and suddenly guns are firing everywhere.
 * Some stages in the Gauntlet (1985 video game) series of games had this, where your shots could either stun or even harm other players.
 * Side-scrolling beat-em-ups, such as Final Fight, also have this. Even ones that did not (such as the Teenage Mutant Ninja Turtles or X-Men games) would have certain special wild attacks (respectively, environmental attacks and hurled enemies) that could harm other players.
 * Doom had friendly fire in co-op mode. Much more famously, any monster with a projectile could incite a riot by accidentally hitting it's allies, which would cause them to forget all about the player and kill each other. They don't really notice if one of their buddies wanders into the line of fire. Quite a bit of Doom strategy is based around tricking enemies into hitting each other.
 * Star Fox 64 had a mission on Katina that, in addition to being a Shout-Out to Independence Day, involved assisting a Cornerian Defense Force base's fighter compliment in fending off Andross' forces. The problem—Andross' fighters are designed nearly identically to the CDF "greenies", and they are both easy to destroy with the Arwing's lasers. Fox's CDF friend Bill will not hesitate to call Fox out if he shoots down a CDF fighter.

Though rare, it's possible for the rest of the Star Fox team to shoot down the CDF fighters as well. "Peppy: Enemy down ... Wait! That was one of ours!"


 * This is a major danger in the Total War series, particularly with siege units.

Web Comics

 * Girl Genius: Agatha gets questioned before she gets helped.
 * In Kevin and Kell, the hunting team takes part in a "blind stalk", in which they hunt in total darkness, guided only by their sense of smell. Vin, a jealous member of the team, tries to get revenge on Rudy by putting prey pheremones on him, and Rudy manages to avoid his teammates long enough to lure in a deer (who mistakes him for a doe) and take his antlers. It initially appears that Vin was mistakenly eaten by Rudy's teammmates, but it is later revealed that he was abducted after stumbling onto the Great Bird Conspiracy.
 * In American Barbarian, Two Tank Omen sends his prisoners toward his enemies, leading them to think the prisoners are his allies -- and they attack them.

Web Original

 * In the TGWTG Team Brawl everybody had trouble determining who's on whose side—one time Nostalgia Critic asked if That Chick with the Goggles was on his side; as she said no he immediately punched her.
 * The first casualty in Red vs. Blue? Church, of the Blue army. The shooter? Caboose, also of the Blue army. Church is not pleased.

Western Animation

 * The Wild Thornberrys had this in one episode, where for a battle, Eliza and Darwin placed two groups of monkeys, one with long tails and one with short, in coconut-shell armor. When the monkeys went to fight, they couldn't tell friends from foes and attacked both, to prove Eliza's point that they weren't that different, and the whole "Tails vs. No tails" thing was completely ridiculous.