Most of the heroes are trapped in a cell, when the door opens and a guard wearing a face-concealing helmet enters.
They prepare for a fight. The guard takes off his/her helmet — and reveals that it's one of their allies.
In the real world, this is illegal for a military operation (under Article 39 of the 1977 First Geneva Protocol), but not when it comes to espionage (which makes it fair game to kill you anyway). Well, that is, unless you take off the uniform before you start firing, and a lot of other small things that our heroes nonetheless forget and our villains disregard anyway.
Note that the guard the hero knocks out is always wearing the right size uniform to fit them perfectly. (Except in subversions, in which case it fits them very poorly). This may or may not follow a Mugged for Disguise.
This occurs so often that Rule Number 1 on the Evil Overlord List is that the Legions of Doom should wear clear helmets. Another rule from that list calls for guards to wear tailored uniforms which will not fit any hero who attempts this trick.
Actually a very common mythic trope called 'Wearing Enemy's Skin' identified by Joseph Campbell. Known examples go back to The Iliad, thus Older Than Feudalism.
In fantasy settings where some sort of magical powers are available, a common variation of this involves actually transforming the hero into one of the enemy.
May lead to Friend or Foe since you do, after all, look like the enemy.
See also The Mole, Trojan Prisoner, Hey, Wait!. On a regimental scale, this is a False Flag Operation. When used with zombies, it's Pretend We're Dead. When the disguised arouses suspicion by constantly insisting on his alliance to the enemy, it's Most Definitely Not a Villain.
A very odd example comes from Super Dimension Fortress Macross and its Robotech adaptaion: Max is trapped onboard a Zentraedi warship and hides in a bathroom. An enemy soldier comes in and Max knocks him out and steals his uniform. The pockets come in handy for rescuing the three human prisoners. The oddity is that Max didn't disguise himself, but rather his Humongous Mecha in the uniform of the equally giant alien.
Not quite enemies, but the Roberia Zuka Club makes their first appearance in Ouran Academy uniforms, one as a male.
Jugo and Suigetsu try this in Naruto using samurai uniforms. They are caught almost immediately.
Bleach: Uryuu and Orihime steal and wear some shihakusho to blend in during the Soul Society arc. Because it's Orihime and Uryuu, an awkward Lingerie Scene is mandatory.
Erza Scarlet in Fairy Tail strips her Edoras counterpart in order to pose as her. Funny how Erza has no problems with stripping her counterpart naked (well at least partially naked).
The group infiltrating Baba Yaga castle in Soul Eater dress in the same way as the Artifact Soldiers with long black robes and white masks. Causes a Crowning Moment of Awesome/Funny when Kirikou decides to reveal himself after wandering around for a while: "Surprise."
During the One Piece G8 Filler Arc, the Strawhats are forced to do this. They are quickly picked out as fakes when the commander gets reports of suspicious unknown Marines wandering around the base.
Bernie says that he is from Australia and says at Christmas time the city was covered in snow-this is possibly understandable as he is a Zeon and may not know much about Earth since he living in a colony.
Heero Yuy in Mobile Suit Gundam Wing is seen infiltrating enemy facilities almost every other episode. He's incredibly brazen about simply knocking out guards and taking their uniforms (which somehow always fit him) and then waltzes right into the enemy base without a second thought. Trowa also does this once.
But they are trained operatives and probably were trained to do this sort of thing for their missions.
In Mobile Suit Gundam 0083: Stardust Memory, Anavel Gato accomplishes the hijacking of the first episode by walking into the hanger while wearing a Federation uniform provided by a confederate. He complains that the uniform's rank pins are too low.
There was one episode of the second TV series where Lupin, Jigen, and Goemon ALL dressed up as Zenigata.
Anyone in the cast might be expected to appear as a disguised version of someone else. Zenigata has even disguised himself as Lupin!
Mary-Maria tries this in Archer And Armstrong to sneak into Area 51. The costume fits fine, but she's still caught almost immediately, possibly because she has a hard time concealing her surprise when she runs into an alien.
Played semi-straight in multiple Astérix stories. At Obelix's insistence, they always look for two enemies, one small and one... medium-sized. Note that Obelix is a big man by any standard imaginable, in case you didn't know.
In an "alternate future" storyline in JLA, Batman spent years posing as Desaad, right under Darkseid's nose.
An extreme example happened in Sin City. Dwight McCarthy had previously been shot in the face, requiring surgery. This ended up changing his appearance drastically. The Big Bad, meanwhile, was planning to see a drug courier. Dwight had the courier killed and took his place in order to infiltrate the villain's estate. Since he was essentially disguised after his surgery, this allowed him easy access to the villain's mansion.
Le Scorpion: Done twice The Angel's Shadow. First Armando disguises himself as a warrior monk as part of a Trojan Prisoner gambit to gain access to the Vatican dungeons. later, he and Hussar disguise themselves as Warrior Monks to steal a shipment of gold.
Since Mandalorians are always wearing their heavily customized armor and rarely taking of their helmets in public, it's very easy for Zayne Carrick to impersonate the evil scientist Demagol to free all of Demagols prisoners in Knights of the Old Republic.
And in a much more shocking case only minutes later, Demagol manages to overpower Rohlan while being put back into his armor for transport, and then drugs him up enough to be in a coma for half a year, during which he impersonates Rohlan.
A villainous example from The Untouchables with Frank Nitti, who briefly dresses up as a police officer to infiltrate the police department and assassinate one of the Treasury Department agents and their informant, along with two police officers who were waiting outside.
Blue Streak, has the main character posing as a cop in order to retrieve a diamond he left in a police building. It grows on him.
A similar example to the page quote occurs in Star Wars: Return of the Jedi, where the bounty hunter that sells Chewbacca to Jabba the Hutt turns out to be Princess Leia in disguise, there to rescue Han. Also, Lando is in disguise beforehand as a mercenary, bounty hunter, or other kind of scum that would generally hang around Jabba's palace. The quote at the top of this page mentions when Luke and Han dressed up as Storm Troopers so they could rescue Princess Leia in A New Hope.
Ends disastrously in Cross Of Iron. German soldiers are isolated from their unit and must wear Soviet uniforms to sneak back to their base. The German guard commander knows about the deception, but has a personal grudge against the returning sergeant, and orders his men to shoot the approaching "Russians".
Parodied in Austin Powers: International Man of Mystery. Miss Kensington and Austin follow a pair of Virtucom employees into the restrooms, one a very tall and lanky man, the other an obese woman, and emerge seconds later in perfectly fitted uniforms.
And spotted in the five seconds between leaving the restrooms and entering the employee's only area.
Indiana knocked out a Nazi officer at a book burning rally, leading to a too-close encounter with Der Führer himself.
Indiana knocked out a waiter on a zeppelin and took his place.
Inverted in Kingdom of the Crystal Skulls where the American army convoy at the start of the movie are Russians.
Jeff Bridges pulls this off in TRON, but because the good guys and the bad are simply color-coded, all he needs to do for a disguise is steal the glowing red trim around a Mook's costume.
The Dirty Dozen won a war game by using this trope. Since the two sides only differentiated by colour of arm-band, it was surprisingly easy ("We're traitors"), although you wonder why the opposing colonel didn't recognize the ones he talked to. The same trick was used in Private Benjamin, using one soldier's non-regulation red underwear to make the red armbands.
Which was why they did their best to ensure that the members of the team that the opposing colonel had gotten a close look at weren't part of the infiltration stage.
In addition, The Dirty Dozen dressed as German soldiers for their actual mission.
In Sahara, a straight version of this allows the protagonists entry to the mooks' base, especially since they've "borrowed" the Big Bad's car.
The Eagle Has Landed points out the illegality of this several times. There is a flimsy rationalisation where they wear their German paratrooper uniforms underneath Polish uniforms, and only engage in actual combat as Germans.
It wasn't an attempt at an excuse, it was because the German CO regarded the deception as Dirty Business and was adamant that if their cover was blown "We will die as what we are; German paratroopers."
In Where Eagles Dare, Richard Burton's team of commandos don Nazi uniforms to infiltrate an enemy castle, purely to get Clint Eastwood running around shooting people in German stormtrooper garb. In a twist, some of the commandos are undercover Nazis, their true allegiances reflected in what they're wearing.
The Guns of Navarone. After turning the tables on and capturing their German Army interrogators, our heroes don their uniforms and escape.
Inverted in two different way in The Dark Knight. First when the Joker's gang try to kill the mayor by taking the place of Police Officers in a parade, and then when they put their masks on civilians who have their mouths taped shut and their hands taped to the fake/unloaded guns. The Joker acquires a nurse's uniform when he goes to pay a visit to Harvey Dent.
A similar inversion occurs in V for Vendetta, when V dresses up bound and gagged employees of the station he's attacking in replicas of his own costume, leading the guards to unwittingly open fire on them.
At one point in Dr. Strangelove, a column of US Army troops is dispatched to storm the Air Force base which has been commandeered by renegade general Ripper. The base troops think they are battling invading Russians:
""You sure gotta hand it to those Commies. Gee, those trucks sure look like the real thing, don't they? I wonder where they got 'em from?"
"Probably bought them from the Army as war surplus. OK. Open up at 200 yards.."
Blazing Saddles. Two Ku Klux Klan members are part of the line of applicants to join Hedley Lamaar's army of Mooks. Sheriff Bart and the Waco Kid lure them away, knock them out and steal their robes. Unfortunately, they forget that Sheriff Bart is black...
Also parodied in Spaceballs. Lone Starr and Barf lure two Spaceballs into their Winnebago, beat the crap out of them, and come out with their uniforms. Barf's is perfectly fitted, even though he's much larger than either of the soldiers...and has a tail...
Subverted in the Swedish film The Third Wave. A hitman who's stalking his target through an anti-globilisation riot kills a riot policeman and steals his gun, uniform and face-concealing helmet. This enables the hitman to get close to the protagonist and execute him with a shot to the head — unfortunately it turns out the gun he stole is loaded with plastic bullets.
The Wizard of Oz. Scarecrow, the Tin Woodman, and the Cowardly Lion are ambushed by three of the Wicked Witch of the West's Winkie castle guards. In the next scene, the heroes sneak into the witch's castle dressed in the guards' uniforms.
The Simpsons Movie. Homer dresses as a hotel doorman to fool a soldier that he is a superior officer. He's only given away when the soldier queries why an order from the President is written on a leaf.
Judge Dredd. While Dredd is on the run from the law, he knocks out a Judge and steals his uniform. It fits perfectly.
In Battle of the Bulge, German soldiers dress as Americans to infiltrate and confuse American attacks. Truth in Television, but the effect was mostly psychological, as the actual operation was a failure.
The third Teenage Mutant Ninja Turtles film has the turtles do this with a justification. The established rules of time-travel in the movie state that in order to go into the past, the turtles would have to swap places- and uniforms- with whoever is holding the scepter in feudal Japan. It just so happened that the ones in the past were the Daimyo's honor guard. Luckily, the ceremonial samurai masks allow the Turtles to disguise their faces as well.
In The Gumball Rally, one of the entries into the race is a Dodge police cruiser with black and white paint job, and magnetic door decals for police and highway patrol logos of every state they would be crossing through.
Moira in X-Men: First Class does a variation when she has to sneak into a strip club. In this case it's "undressing" like the enemy...much to her partner's surprise.
In Posse, Jesse poses as a Ku Klux Klan member to rescue Papa Joe and Obobo.
Serenity: It was said about Reavers that they skinned people alive, raped them repeatedly, ate their flesh and then murdered them. They kept the skins as trophies, their spaceships were covered with blood, and they even tied whole corpses to the front of their ships. Knowing this, Mal employed it in the Big Damn Movie, using his murdered friends as disguise. The late Shepherd Book had the "honour" of being the centre on Serenity's nose. It worked, as they went through the Reavers' territory and back, and survived.
Mocked in the MS Ting of Space Mutiny when the beefy hero knocks out a smaller man in a tight-fitting uniform and then changes into it.
In G.I. Joe: RetaliationStorm Shadow pretends to be Snake Eyes so that he can break into the German facility and free Cobra Commander. Also Jinx disguises herself as a Cobra red ninja to get close to the Cobra Commander, Firefly and Zartan along with Storm Shadow, who was going after Zartan because he murdered the Hard Master.
The Chronicles of Riddick: Riddick disguises himself as a stormtrooper to get inside the Lord Marshal's ship. Subverted, though. Dame Vaako, one of the only Necromongers who actually knew what he looked like, still recognized him—she just decided to let him have his shot. He also had to kill a couple guards who caught onto him.
Bad Reputation has a dark variation, where the avenging heroine kills her way up the Girl Posse hierarchy during a costume party organised by the lead rapist, taking their costumes in turn to get ever closer to the villain.
The Baudelaires from A Series of Unfortunate Events unintentionally do this in The Hostile Hospital when they disguise themselves as doctors and are mistaken by Olaf's associates for the two powder-faced women who are also disguised as doctors.
Even King Arthur makes use of this trope, with tragic results: The brothers Balin and Balan go their separate ways, have adventures, defeat their enemies, plunder the bodies, and put on the better armor. By the time they meet again, their armor and shields make them unrecognizable to each other. Tragedy ensues.
Similarly, in Homer's The Iliad, following the siege of Troy, the Trojans took the armor off dead Greek warriors and put them on in an inevitable resistance. Any poor Greek who thought he was joining his allies was racing towards his own death.
In The Aeneid, the tactic is shown from the Trojan's viewpoint; it succeeds for a while, but the Greeks figure out their ploy, and the Trojan archers have no idea that the band of soldiers is made of their allies, so Aeneas loses a few men to the Greeks and to friendly fire.
In Victor Hugo's Les Misérables French National Guard uniforms are worn by several revolutionaries and a jealous adoptive father to pass through National Guard lines into the barricades.
In Terry Pratchett's Discworld novel Jingo, after being dragooned into spying on the Klatchians with Lord Vetinari, Fred and Nobby attempt to steal some Klatchian clothing in order to blend in. Unfortunately, since it's dark out they only end up beating each other up, and the men they lured into the alley rob "them" instead.
Nobby was never on the losing side of a battle when he was a quartermaster, the reason being that he'd sneak off and change into a stolen enemy uniform the moment his lot started losing. Due to his habit of selling all his army's weapons and armor (often to the enemy) this happened frequently: enough that experienced generals kept an eye on what he was wearing to see how the battle was going.
In Open Sesame by Tom Holt, two main characters prepare to jump on a pair of guards, realise the outfits wouldn't fit, so give the guards their measurements. The guards, who are bound to narrative rules, walk off and send in two more guards, who are promptly knocked unconscious, letting the main characters steal their perfectly fitting costumes.
Used in the Robin Hood ballad where he faces Guy of Gisbourne, wherein after killing Gisbourne, Robin fakes his death by switching clothes with Gisbourne's corpse and mutilating the corpse's face so it's unrecognizable. There are also occasions on which Robin or his men dressed as the enemy without needing to employ fatal measures.
Done twice in the 1973 Disney Robin Hood cartoon. Robin disguises himself as one of the sheriff's vulture guards, and Little John as the sheriff himself.
Done in C. S. Lewis's final Chronicles of Narnia book, The Last Battle. Since the enemies are thinly disguised Arabs, the deception includes wearing blackface. One wonders how the upcoming film adaptation in going to handle this in these more PC times.
There is a very clear difference between blackface and using makeup and other chemicals to darken ones skin for espionage purposes.
Captain Horatio Hornblower R.N. uses this once or twice - not enemy uniforms specifically, but flying the enemy flag on approach, before whipping it down and running up the Union Jack before opening fire. Not an uncommon tactic in naval warfare of the time but to be legal, the correct ensign had to be flying prior to commencement of active hostilities.
Judge Dee and his lieutenants are constantly disguising themselves as assorted members of the underworld. Tao Gan actually IS a member of the underworld, retired.
Done at least three times in the Redwall series: Brome of Noonvale dresses up as a searat, Jukka Sling (a squirrel) shaves her tail and dyes her fur to pass as a Blue Horde rat, and Midge Manycoats designs elaborate vermin costumes for himself and Tammo. On one other occasion, Mariel and friends are wearing stolen searat clothes while on board a ship, are seen by a helmsrat on another ship, and he mistakes them for other searats even though they weren't expecting other rats to be around and aren't actively pretending to be such.
In J. R. R. Tolkien's The Lord of the Rings, Frodo and Sam disguise themselves as orcs during the last leg of their journey. And no, it doesn't fit either of them right. Good thing it's dark.
Used in The Silmarillion as well, when Beren, Finrod, and his soldiers disguise themselves as Orcs. It doesn't work.
And later in the same story when Lúthien disguises herself as a bat and Beren as a werewolf to infiltrate Morgoth's citadel, this one does work.
In Sandy Mitchell's Ciaphas Cain novel Death Or Glory, Cain and his group travel in stolen ork vehicles. This does distract the orks, but when they come into firing range of an Imperial force, they nearly get fired on before they manage vox communication, and even then the force demands proof.
In Graham McNeill's Warhammer 40,000Ultramarines novel Nightbringer, an eldar Pirate approaches an Imperial vessel broadcasting that it was the Gallant, an Imperial ship. Unfortunately for him, someone onboard knew that particular ship had been destroyed five years earlier.
Subverted in "The Wonderful Story of Henry Sugar" by Roald Dahl. The titular main character has found a way to beat the casinos, but he gets cocky one night and the owners send goons to his hotel. The bellboy comes to warn him (not out of altruism, but in the belief that it doesn't hurt to have the gratitude of a man who just won a stack of money) and offers to pretend that Henry overwhelmed him and stole his uniform to sneak out of the hotel.
The Wraiths of the Star Wars: X-Wing Series have reason to do this a few times - dressing as Space Pirates in some situations, stormtroopers in others. As pirates they need no ID, but they run into some trouble as stormtroopers without the proper passwords.
Bones of the Hills opens with a group of Mongols under Tsubodai engaging a regiment of Russian knights. Jochi's argan manages to kill a detachment of them, then steal their armour in order to attack the main force.
In Pendragon: The Merchant of Death, Loor and Press dress as Bedoowan knights.
The Belgariad has a city filled with Grolim priests, conveniently wearing identical black cloaks and steel masks to hide their appearances. The good guys... obtain... a few, helping them get around the city despite the bloody stab holes in their new clothes.
Played with during one sequence in the Belisarius Series, the title character has to escape from a city full of bad guys out for his blood. He needs a uniform to escape, but with the thousands of guards looking for him, he has difficulty finding a uniform that's both the right size, the right type, and being worn by a guard he can take out quietly. Several times he pulls a Bavarian Fire Drill on guards he runs into to get them to search someplace else. Eventually he finds the uniform, and makes it out of the city.
Done a couple of times in Harry Potter, aided by Polyjuice Potion:
In Chamber Of Secrets, Harry and Ron disguise themselves as Malfoy's best friends Crabbe and Goyle to question him about the Chamber.
In Deathly Hallows, the Trio Polyjuice themselves into three random Ministry employees to sneak into the Ministry of Magic. It works, but it's Subverted in that the "random" part causes endless problems: Hermione and Ron are stuck in the jobs of their disguises not to blow their cover, and they have to bluff their way though; Ron's is man whose wife was on trial that same day]]. Luckily Harry realized quickly his disguise was a Death Eater. As all hell breaks loose he just starts giving orders and threatening people who try to stop them, getting a bunch of Muggleborns out through the front door. Then the real guys, they didn't knock out, return.
And again when Hermione and Ron pretend to be Bellatrix and some Death Eater-friendly foreigner to break into Bellatrix's Gringotts vault. Subverted again when the Goblins had been warned that someone may try to enter the vault, and Hermione having Bellatrix's wand blows her cover instead of helping it.
In David Eddings' Tamuli series, some of the heroes disguise themselves as mercenaries (complete with different faces) to get into Scarpa's army.
MAD Super Special Fall 1980. In The Moronic Woman (a The Bionic Woman parody), Jammy Summons (Jaime Sommers) and Oscular (Oscar Goldman) have infiltrated the enemy palace. In order to get a guard's uniform for Oscar to wear, Jammy walks around knocking out guards until they find one who is Oscular's size. Oscular puts on the uniform and they go to the office of the dictator.
Inverted in the Paladin of Shadows book A Deeper Blue, where mooks disguise as a friendly tactical team and later DEA agents.
A common tactic of Company C in the American Civil War novels of J.T. Edson. Sometimes it was not even necessary to dress in enemy uniforms as the only difference between the Federal and Confederate uniforms was the colour, so in near darkness it was quite easy to pass yourself off as a member of the opposing side.
Hogan's Heroes used it a lot, to the point where they have an entire wardrobe full of various German uniforms of differing ranks and positions tailored to fit each of the Heroes. In one notable case, a pair of British spies impersonated an SS film crew to film the camp unnoticed.
Subverted in the Doctor Who episode "The Runaway Bride"; The Doctor steals a guard's uniform and uses it to infiltrate a secret chamber — but the enemies aren't fooled for a second.
In at least three stories, the Doctor and his allies manage to disable a Dalek, remove the mutant inside, and substitute one of their number. How a whole person fits in there, when the mutant that came out is not much bigger than a human head, is never made clear.
At other times, people have fooled Daleks by dressing up as Robomen or Dalek troopers. In the former case, the deception was falling apart, but was saved when the Dalek were distracted by an attack by people without disguises. In the latter, the deception worked until they were seen in the self destruct chamber.
The chunkheaded hero from the MST3K-lampooned Space Mutiny beats up a guard who's easily thirty kilograms and 25 cm smaller than he, yet his stolen uniform fits perfectly.
Crow: "So he fits into a suit that was restrictive on a really small man?"
And within minutes, the disguise is blown at the first checkpoint he comes to. A subversion, perhaps?
Another example from MST3K: The episode Samson vs. the Vampire Women features a mook trying to disguise himself as a wrestler that the titular hero is about to fight — only the mook has a substantially different body shape from that of the wrestler. (This fact is painfully obvious since the wrestler — and the mook who takes over for him — wears nothing but tights and a mask.)
In Babylon 5 the attempt to free Sheridan on Mars is blown when one of the infiltrators is seen to have fresh blood on his uniform (leaking from a bandaged stab wound obtained previously).
The rebels often borrow the Visitors' uniforms in the miniseries and series V. In the original miniseries, Donovan takes the uniform of a much smaller, female Visitor, who explains that it will stretch to fit him. Apparently they're one-size-fits-all.
Farscape. John Crichton infiltrates a Peacekeeper base disguised as an officer (a ploy that had worked successfully before) but encounters Big Bad Scorpius for the first time who casually says "Guards, that man is an imposter. Seize him." It turns out that Scorpius has the ability to see the heat signature of others, so he could tell Crichton wasn't Sebacean.
John keeps wearing the Peacekeeper uniform throughout the season, though, partially because he doesn't exactly have a whole lot of changes of clothes with him and partially because it makes him look badass.
He actually has two Peacekeeper uniforms in that season: the captain's outfit (red and black) which was left behind on Moya by her former crew, and the commando captain's uniform which he wears to infiltrate the base, while impersonating its previous owner. Aeryn also acquires a cast-off female commando's uniform at the same time, which counts as this trope because while she was a Peacekeeper, she was never a commando.
How often it actually works depends on how merciful the writers are at the time. In "Liars, Guns and Money Part 1," Crichton and Aeryn knock out a couple of guards in the middle of their bank heist, but are ambushed by more guards before they've even finished pulling the clothes off. Crichton flees with a half-hearted "Uh... they went that way!" to the new arrivals.
In Volume 4 of Heroes, Peter is all set to try this after knocking out one of the black ops agents, until Tracy points out to him what a stupid idea it is (he still puts on the guy's uniform, though, because why say no to free Kevlar?). In a later episode, Sylar does the exact same thing, with much more success, although he has much more experience in performing that particular trick.
Also used by Hiro and Ando, but Hiros glasses and lack of English give him away, forcing Ando to stun the other soldiers with his power.
Lampshaded and subverted in the episode "Patterns of Force". Kirk and Spock attempt to do this on a Nazi planet to try and get close to the Fuhrer, who happens to be a Federation citizen. Not only does Spock question the logic of doing so (only to concede that it is in that circumstance), but they end up getting caught shortly afterward because of a lapse of protocol they didn't know about. Double-subverted when they try it later and manage to succeed.
McCoy later on beams down dressed as a Nazi military doctor upon Kirk's instructions—implying that the Enterprise has a whole wardrobe department stocked with historical uniforms and other clothing (this was before the replicator, which would have made more sense). Because Kirk and Spock have already been caught, and McCoy's boots don't fit quite right, McCoy is captured as well.
In "The Enterprise Incident" Kirk dresses as a Romulan (including getting an "ear job") to infiltrate a Romulan ship and steal its cloaking device.
Kirk and Spock also disguise themselves as gangsters in A Piece of the Action.
In the Highlander episode One Minute to Midnight. Methos almost lampshades it with the line "You know, this 'Bad guy just my size' routine never works, Macleod. " Less than a minute later, he proves it DOES work, by donning an Irish accent along with the bad guy's coat and shoots the other henchmen.
A Space Opera example occurs in the episode "Asteroid" of the BBC series Hyperdrive. The Camden Lock leads a pursuing alien fleet through the upper atmosphere of a gas giant so that the planet's atmosphere will destroy the pursuing ships. Following this maneuver, the crew finds that one of the alien ships has survived with its sensors out of commission and is hailing them to determine if they are friend or foe. The crew pretend to be aliens right up until they open fire.
Rotten Town, LazyTown's evil counterpart has the people dressed like the "evil dude".
Game of Thrones. Sandor Clegane is riding up to the Twins when Frey soldiers suddenly turn on their Northern allies and massacre them. Sandor escapes the carnage by picking up a discarded Frey banner and riding out of the camp holding it.
Discussed and Deconstructed in Young Indiana Jones in the "Prisoner of War" episode when Indy is sent to a prison during World War One; a guard warns prisoners against any means of escape, warning many of them wouldn't work in this prison. He does bring up that anyone disguising themselves as a guard would be shot as a spy.
Red vs. Blue. To fool the Reds into thinking they have more than one freelancer, Tucker and Caboose are ordered to draw the Red's fire while wearing black armour. How do they get black armour? By jumping through a very experimental teleporter.
Later in the series, Simmons defects to Blue team, originally because he was shunned by the others, but when Church returns, he begins a charade to try and learn information. To get the effect, he paints his armour blue (although he misses a few spots), but it is subverted in that Church is not fooled by this disguise for one second. Church even takes advantage of the situation by making Simmons do chores and make him lunch...
Earthdawn supplement Prelude to War, adventure "The Dragon's Daughter". The PC's must dress up as members of a mercenary band to infiltrate the Theran Behemoth Triumph.
In Exalted you have charms that make any disguise you want to wear, perfect and infallible to non-magical means of perception. A Night caste Solar can infiltrate anything he wants.
In The Saboteur, Sean is able to knock out a Nazi soldier and take their jacket. However he must only stealth kill to get the outfit as the blood would immediately give him away. Also it is noted in the How To part of the game that Sean reeks so much of booze and smokes that if he was to get to close to the Nazis for too long they will be able to spot him out.
That, and he only ever takes the jacket and headgear, and leaves the pants and boots alone. While it's justified for a few reasons (There's usually too much risk to spend time putting on the full uniform and, in any case, the guy's likely soiled himself as he goes down - Not pretty.) it does result in him being out of uniform from the waist down. Also, for whatever reason, Sean also refuses to wear the armband as well.
In Final Fantasy VI, when Locke infiltrates South Figaro, you can switch between Locke's normal outfit, a merchant outfit, and an Imperial guardsman outfit; if you rescue Celes while wearing the guardsman outfit, Locke hides in plain sight as the enemy soldiers walk out, and there's a Shout Out to Star Wars during the actual rescue.
It is entirely possible to beat Fallout 2 in around eleven minutes, by swiping a suit of enemy power armor and waltzing into their nigh-impenetrable fortress.
This trope is also featured in the original, where you can wear robes and almost everyone in the Cathedral will assume you're one of them.
Metal Gear Solid 2: Sons of Liberty featured an enemy uniform that could be worn by Raiden. Konami deceptively hid his presence in the game by only showing him dressed in this disguise for the game's promotional materials, which may have had something to do with the incredible backlash against the character when he was revealed to be the main protagonist. It also averts the Universal Uniform trope, if only in the mildest way: the uniform is (allegedly) too big for Raiden, and if an actual guard touches him, it instantly falls off, mandating a tactical withdrawal.
And in Portable Ops, you recruit your squad from captured enemy soldiers, so nearly all of them are dressed similarly if not identically to the guards found around the levels.
Metal Gear Solid 3, in turn, requires you to knock out a GRU officer and steal his uniform. The officer in question looks suspiciously like Raiden.
When playing in most of the uniforms, you can't do anything too weird (rolling around, beating up guards, etc.) or you'll be caught. As the GRU officer, you're explicitly allowed to do anything you want, because "Raikov's just like that". It's probably because no-one wants to tick off the villain's boyfriend.
Earlier in MGS 3, Snake is required to dress in scientist garb twice, once to infiltrate a warehouse for a rescue mission and the other to infiltrate a military base...for a rescue mission for the same person. If one of the actual scientists sees you, they will stare at you for a while before sounding the alarm since they don't recognise you. Hence, turn away so the scientists can't see your face. How this works in the first place I have no idea, since Snake doesn't look Russian at all, and he's far from clean-shaven, and this works as normal while missing an eye (Which of course, isn't reflected on the ID badge).
"Gee, that's weird. I don't remember the American scruffy-looking scientist with the eyepatch."
Also Meryl dressed as one of the guards in the first MGS.
In Metal Gear Solid 4, you dressed as... technically, potential friendly. Rebels, while seeing you in standard uniform, will warily move around you, even attacking you if you got too close. You had to build relationship with them by giving them items and attacking PMC troops. However, wearing their uniform will instantly give you their trust, which makes your life easier.
Several missions in City of Heroes and City of Villains give you an enemy disguise as a temporary power, allowing you to move freely among enemies while it's active. Of course, you don't have to use it...
...and some of us preferred not to. Especially when you disguise yourself as a cybernetic freakshow◊.
Partially subverted in Final Fantasy VII. The heroes sneak aboard an enemy ship wearing the enemy army's uniforms. However, one of the teammates, a lion-like quadruped, stands out quite a bit as he staggers about trying to walk on two legs. Also, his tail is showing.
Which, come to think of it, could easily be a shout-out to The Wizard of Oz, only explaining why that makes sense would involve a lot of Fridge Logic.
Spoofed in Final Fantasy IX. Zidane and Blank steal Pluto Knight armor, and comment about the armor being sweaty, stinky, and breadcrumbs being in the pockets.
Chrono Cross features a particularly goofy example. Access to the inner sanctum of Viper Manor requires the heroes to dress up in the uniforms of three guards. Three radically different-looking party members will all look the same, but pretty puffy pink pooch Poshul doesn't even bother putting on a uniform. Nobody seems to notice two guards hanging out with a neon-pink talking dog.
The cartoonish class-based First-Person ShooterTeam Fortress 2 gives this ability to the Spy class, who is able to dress as any friendly or enemy character class. Opposing players see the model he is disguised as, but his allies see the Spy model in a cheap cardboard mask of the class he's impersonating.
He also steals the username of one opponent who is playing the class he's impersonating. So if you see another you running around, raise an alert right quick.
Red Faction features a slightly more realistic version in which your disguise will only stand up if you avoid getting closer than about ten feet to an enemy guard/camera for longer than around a second (you're basically public enemy #1 at this point).
Even though it may be faster to run through guns blazing, you still need to switch clothing. The second time around, feel free to skip the disguise.
In Knights of the Old Republic, you have to do this twice: once, you wear a stolen Sith uniform to gain access to the lower levels of Taris, and later you have to wear Sand People's robes to safely approach a Sand People Enclave.
Subverted on both occasions. On Taris, the uniform only works with the upper lever guard. The lower guard demands that you show him the proper papers before you pass. On Tatooine, the Tusken Raiders realise you're an imposter on closer inspection...somehow...
The EU states the since Tuskens always wear robes all the time they tell each other apart with their gaderffiis (and banthas) since no two are alike, so to them you're basically wearing a name tag.
You later trade the Sith uniform to the Hidden Beks leader who planned to invoke the trope by disguising some of his gang in case the Sith tried to take over the Lower City. Unfortunately, Carth was right in that the Sith decided to go with the "much more grand and deadly" idea of razing the planet
Planescape: Torment allows the main character several methods of escape from the beginning "dungeon"—including stealing one of the "guard's" uniforms or getting oneself disguised as a zombie (which are commonplace). Both of which result in the only change to the character's outward appearance (aside from what kind of weapon he's holding) in the entire game.
Used twice in Metroid Prime 3: Corruption. Early in the game, shapeshifter Gandrayda gets the drop on a squadron of pirates by disguising herself as one. Later in the game, after her Face-Heel Turn, she lures Samus into a trap by pretending to be a Galactic Federation Marine.
A major part of the Chapter 2 story mission in Final Fantasy X-2 revolves finding and stealing the appropriate mook-gear to infiltrate the enemies hideout in Guadosalam. Despite the fact that this is mentioned during several of the battles with the mook's bosses, they never realize that those stolen uniforms are being used for infiltration even when you're talking to the mook's bosses in the base.
To be fair, the uniforms are fully covering and Yuna says only a few lines while in disguise. Also the Syndicate leaders aren't the smartest people in the game anyway.
This is one of the most common gameplay mechanics in the Hitman series. No matter who you knock out, their clothes will always fit, and the others will never notice that their friend is now a bald white guy with a barcode tattooed on his head.
As much as the series loves prison break chapters, only Fire Emblem: Path of Radiance has a character pull this off: Naesala, to rescue the captured heron princess Leanne. But how did he hide his wings? I know he had a cape, but still!
In Grand Theft Auto: Vice City, one mission includes you and Lance Vance beating up two cops and taking their uniforms. They fit perfectly, although Tommy Vercetti complains that it's "a bit tight around the crotch".
One of the missions in Bully has Jimmy put on an Aquaberry sweater and posh hairdo to infiltrate the Preppies fraternity house.
Funny that he can change his length of hairstyle at will, despite having a buzz cut. The magic of video games!
And don't even ask about some of the disguises that Leisure Suit Larry fits in. Granted, it's all parody, but still, the Vegas-style showgirl outfit????
Happens multiple times in MOTHER 3, and two times the enemy helps you disguise yourself. When the team infiltrates Thunder Tower, the Pig Masks mistake Lucas for their commander, and give him his "usual" clothes, and Pigmask outfits for the rest of the party. Later, Lucas and Boney infiltrate the Chimera Labs and are given Pigmask uniforms to complete their masquerade as part-time workers. Given who the aforementioned "commander" is, this makes sense.
Some missions in Medal of Honor games require dressing up as a German officer and displaying credentials to gain access to valuable targets like a submarine or rail gun. Someone always eventually sees through your disguise.
Kinda parodied in Rising Sun where you have to steal one officer's uniform, and upon showing the pass for the officer on railroad station's door, looks at you for a moment and says: "Damn, that just must be other photo of you!" and opens door for you.
In Light Crusader, you get a goblin costume that you use to sneak past guards.
In Paper Mario: The Thousand-Year Door, Princess Peach dons an X-Naut uniform in order to get information from her captor. The disguise is flawless, despite X-Nauts being around half of Peach's height.
In Perfect Dark Joanna was able to dress up like a Scientist, and a Flight Attendant in order to sneak into a Lab where an Alien was about to be dissected, and Air Force One.
Worldof Warcraft features a few opportunities to disguise yourself as an enemy, usually only for the duration of a quest. There's also a very rare item that allows the player to disguise themselves as the enemy player faction (A Horde Player becomes disguised as the Alliance and an Alliance player becomes disguised as the Horde, respectively). Creative-minded players have even gone to the trouble of purposefully not completing those quests where a Disguise item is provided, and have used it in various ways, such as pranks, getting the drop on an enemy player (which is playing this trope straight), or just for fun.
This occurs in the beginning of Half-Life 2. Barney is officially a Civil Protection officer, which leads Gordon (and by extension, the player) to initially believe he has been captured when Barney helps him escape.
In Call Of Duty 4, your squad briefly takes the uniforms of enemy soldiers to ambush a convoy. Griggs comments that "you look like a clown in that outfit."
A major controversy erupted over a level in 4's sequel, in which the player goes undercover as a terrorist, infiltrates the villain's group, and then helps (optionally, the player can choose not to) open fire on dozens of innocent civilians at a Russian airport. However, the villain appears to be too Genre Savvy, abruptly killing the undercover player at the end of the mission, framing the US for the attack and prompting a Russian invasion of Washington DC. However, the villain was actually told of the undercover player by US General Shepherd himself, who wanted to start a war in which he could use the full power of the US military.
In StarFox: Adventures, after a certain point in the game, Fox gets a Sharpclaw disguise, which, although it allows activate certain pads, pick up certain objects, and not get attacked by real Sharpclaw or the floating drones, leaves him unable to use Krystal's staff. He can, however, get close, disengage the disguise, and maybe strike before the enemy can react. If the enemy sees him change, he will simply walk toward Fox, pushing him around instead of attacking, or, in Scales' case, posing threateningly while he waits for you to press A.
The player character, Karl, an American soldier is dressed in German gear (as a sniper this is very useful) in Sniper Elite. Unfortunately he's mostly fighting the Russians.
Prototype takes this one step further: not only can you dress up as the enemy, you can become the enemy by consuming them whole.
In Pokémon HeartGold and SoulSilver, in which the main character has to dress as a member of Team Rocket to infiltrate the building they just took over. It actually makes sense that it works. Too bad your rival shows up and blows your cover.
In the Pokemon Special manga, this happens a bit more often. Red dresses as a Rocket to infiltrate one of their ops, but Blue robs him of the outfit at one point deep in enemy territory. Diamond also infiltrates the (more retarded than usual) Team Galactic airliner by dressing as one of them. Gold, of all people, even gets away with disguising himself as Guile Hideout - and because it physically resembled his own cursed suit of armor, this actually gets Archie killed! That wasn't part of Gold's plan, of course, but a welcome fringe benefit nonetheless.
Looker, being a spy and all, does this a few times.
One part of the 2007 fangame Pokémon Illusion requires the player to put on a Team Rocket outfit for infiltration purposes. Then Giovanni recognizes them when he addresses the grunts. Whoops!
In Tales of Symphonia the healer of the group and the failure assassin Sheena, if you chose the hard path dress up as Desians to infiltrate one of their ranches to destroy.
EverQuest: Secrets of Faydwer features a task called "Disrupt the Workshop", in which you are required to disguise yourself with a clockwork gnome illusion. This allows you to walk freely throughout S.H.I.P. Workshop without drawing the attention of most of its otherwise hostile inhabitants. Since you are allowed to refresh the disguise at will as long as the task is in your journal, this can be abused to make any endeavors in the zone much easier.
Formerly shown in the trope's image example, we have Milanor of Yggdra Union. During the Save the Princess chapter of the story, there are two battlefields where he dons Imperial armor in order to infiltrate the fortress of Karona and look for Yggdra (the armor is given to him by a would-be traitor who's trying to get a specific prisoner freed). During the former battlefield, Milanor cannot do battle when disguised because the stationed soldiers all recognize him right away; your other forces have to sneak in after him and cause a distraction to let him slip through. His disguise is also seen through immediately by the sole guard of the prison, but because there's only one of her and you're between her and backup, you're able to fight and defeat her instead of getting a game over.
The original Castle Wolfenstein allowed you to dress as a German soldier to sneak by the Nazi guards. Unfortunately, the SS Soldiers will immediately recognize you anyway.
Fallout: New Vegas lets you disguise yourself with certain factions' armor, which causes those factions to think you're a friendly, but also makes their enemies hostile as well. In other words, walking into an NCR Outpost wearing Caesar's Legion armor (or Primm while wearing Powder Ganger armor) is a good way to get yourself ventilated. Also, the Elite Mooks of the faction will see through your disguise if you get too close.
This was after the aversion in Fallout 3; on the way to see the President in Raven Rock, it's very possible to mug an isolated Enclave soldier for issue weapon and armor. They don't react any differently, and your impersonation isn't lampshaded or handwaved.
Assassins Creed II has dressing as a guard and carrying a chest to meet Rodrigo Borgia. Assassin's Creed: Brotherhood also has a mission that involves dressing as a guard and carrying a chest to meet a Borgia, as well as dressing as a Frenchman alongside Bartolomeo's men to infiltrate the French camp.
Early FPS/RPG Strife has a mission were you need to locate an officer's uniform before you can infiltrate an Order base without setting off all the alarms.
Command & Conquer Red Alert 2 and 3 have the Allied spy unit which can do this. The Imperial Sudden Transport from Red Alert 3 can disguise itself as an enemy vehicle.
Starcraft II: Wings of Liberty features a Zerg organism, the "Changeling", which is capable of assuming the identity of the enemy race, clothes and all, and do some recon. Units won't be able to spot or attack the Changeling without the player's direct intervention in the form of explicit order to attack what appears to be an ally.
The usefulness of Changelings has been somewhat devalued as of Heart of the Swarm, due to, of all things, an Interface Screw. In all previous incarnations of StarCraft, you could click on any unit and select it. As of HotS, you can only select units you control; clicking on an enemy unit accomplishes nothing. So when you band-select all your Zealots and one of them follows the others around but is mysteriously un-selectable, you know exactly what's happening.
Space Quest has you do this in homage to the Stormtrooper disguise in Star Wars. Later, you lose the helmet and blow your cover.
Covert Front has a scene where Kara has to dress in German officer's clothing so that she can drive past a German checkpoint safely. This all hinges on none of the soldiers getting a good look at her and realizing that she is not only a woman, but also the spy whose photograph is plastered over all their bases; once in Lisbon, Kara still has to avoid all the soldiers guarding the roads.
In Ys II, Adol gains a magic that transforms him into a Roo. This lets him gain entrance to the Solomon Shrine, and makes other monsters talk to him rather than attack him.
The Elder Scrolls V: Skyrim allows certain races to do this for the quest where the Dragonborn must infiltrate and search the Thalmor Embassy, by wearing a set of Hooded Thalmor Robes. An Argonian or Khajit Dragonborn is found out almost immediately ("Wait, is that a tail?"), but a human or elvish (or orc) Dragonborn can sneak through with relative ease, and an Altmer Dragonborn can actually order the Thalmor out of the way. What's interesting is that this isn't even hinted at anywhere in the game, leaving it entirely up to the player's initiative.
In one of the Civil War quests, the Dragonborn poses as an enemy courier to deliver disinformation (forged documents) to an enemy commander. If s/he is wearing an enemy uniform, the commander will accept the documents without question. (If s/he is wearing anything else instead, the commander will question it, but will accept the explanation: it's easier to blend in without the uniform.)
BioForge: When wearing the enviro suit, the station personnel will leave you alone unless you attack them.
One level of the Quake II expansion pack Ground Zero lets you wear a "Strogg Uniform" allowing you to wander around the Strogg facility without any of the Strogg noticing, as long as you don't shoot at anyone. It works until you pass through a bioscanner which blows your cover.
In the Syphon Filter series starting with Omega Strain, the player can take enemy outfits from corpses, and some missions start you out with or require you to obtain a disguise.
Subverted in Anti Hero For Hire. While the protagonist wears a Mantis Agent's outfit, which conceals the individual from head to toe, the Mantis use electronic and DNA based identification systems, and is immediately recognized as an intruder.
Subverted (notice a pattern?) in Another Gaming Comic - the player characters done cultist robes, but the Genre Savvy villain has magicked said robes to alert him and all of his other cultists to the fact.
Sojueilo, Thomil and Mijuu of Juathuur disguise themselves as Solluu in order to infiltrate Erab Adur.
In The Dreamer, Alan dresses up as a redcoat to rescue Beatrice from Gen. Howe's ship.
Inverted and played with in Our Little Adventure. While Brian Souballo doesn't impersonate any of the heroes of the story, he magically impersonates a neutral wizard he tried and failed to recruit to his side. The Emperor goes to the very elven metropolis he's planning on invading and commits a mass murder while disguised as the neutral wizard. This way if the wizard goes to the city to warn the elves, he will instead be arrested.
In The Gamers Alliance, the heroes of the Alliance use stolen black cleric robes to infiltrate a city held by the Clergy of Mardük during the Great War. The plan would've been a smashing success if one of them wasn't a mole working for the clergy and leading them into a trap.
Spoofed in an episode of Family Guy, where Peter beats up a bellhop and robs him of his uniform (which doesn't fit) only to be informed that the person they're after doesn't know who they are, making that unnecessary. Fortunately, another man wearing the exact same outfit as Peter happens to walk by.
In "Road to Germany", Stewie, Brian, and Mort stole Nazi uniforms (with Mc Cain Palin button on Stewie's uniform) to infiltrate a nuclear facility.
Spoofed in an episode of The Simpsons, in which, while trying to sneak out of Vegas away from their new wives, Homer and Ned Flanders pull two janitors into a broom closet to rob them of their uniforms. Punching sounds are audible, and the two janitors walk out content, leaving a beat-up Homer and Flanders.
Similarly, in "Burns Baby Burns" Homer and Larry Burns are fleeing from the suddenly half-competent Police (who believe Homer to have kidnapped Larry), and duck into a costume store. The viewer is then shown a man who looks quite a bit like Homer, dressed as an organ-gridner leaving the store with a vaguely Larry-shaped man dressed as his monkey. The subversion comes when we see the clerk look into the store's bathroom, where Homer and Larry are actually hiding: "Either buy a costume or get out, fellas!"
A similar thing happens in Scary Movie 4, where the would-be clothes stealers are themselves beaten up and have to settle for garments on a clothesline instead.
Played straight in the first half of season 3 when the Gaang travels though the Fire Nation in local outfits, change their hair styles and Aang even uses a fake name (Kuzon; and when Sokka doesn't he's given away instantly).
Also near the beginning of Season 3, Hakoda uses a Fire Nation ship (with some of the Gaang's allies dressed in Fire Nation armor) in an attempt to blend in when sailing through Fire Nation waters. Aang almost believes he's been captured by the Fire Nation (having been in a coma for a few weeks). Hakoda spoils the ruse when he mentions getting orders from a commander currently on leave.
Also in "The Boiling Rock", Sokka and Zuko disguise themselves as prison guards. Cue icons of Suki delivering Princess Leia's line.
Dressing as the enemy gets Sokka into to trouble twice with the very people he was trying to rescue, by not identifying himself to them. Suki punches him into the door, Hakoda almost does.
Subverted in the season 1 finale, when a Water Tribe warrior uses antique Fire Nation armor to infiltrate the flagship, and when he tries to kill the admiral, he gets offhand backhanded. Although that was probably less because of a failure of his disguise than because he pulled off his helmet and declared himselfbefore attacking.
Zuko plays with this trope near the end of season 1, disguising himself as a Fire Nation soldier to infiltrate Zhao's ship (played with because he is still an enemy at this point, but he's disguising himself to get past his enemy).
Danny and Tucker briefly disguises themselves as two ghost henchmen in Danny Phantom to rescue Sam from marrying a ghostly prince, much to her dismay as she was orchestrating her own method of escaping just fine.
Done in the "Baloo Thunder" episode of TaleSpin with Baloo, Kit, and their friend Buzz the inventor taking the place of a Corrupt Corporate Executive 's three security guards in order to return a top secret prototype helicopter to its rightful home. Baloo perfectly fits into a uniform even though he's at least twice the size of the largest guard. Also, the bad guy doesn't notice his feline guards replaced by two bears and a bird until it's too late.
In an episode of Quack Pack, Dewey and Louie don the clothes of the human henchmen of the stock villain of the week (some weird pale-faced woman bent on world domination) in order to rescue their brother Huey and foil her evil scheme. Once again, the bad guy in question doesn't notice the species difference.
A common occurrence in the various G.I. Joe series.
It's particulary common in G.I. Joe: Renegades, as the Joes (who are a small group of outlaws in this incarnation]]) frequently inflitrate COBRA Industries to try and find evidence of the company's crimes, and their Cool Car is a stolen advanced COBRA truck, that can change colour and marking to blend in as any truck.
Played so painfully straight in one episode of Mighty Max. Needing to dress as members of a cult, Max, Norman, and Virgil mug some average-size cultists and steal their robes. It's worth noting that Norman is over 7 feet tall and heavily muscled, Max is a skinny kid, and Virgil is a four foot tall chicke...er...fowl. The robes, of course, fit perfectly.
Samurai Jack did this in one episode, killing one of Aku's demon soldiers and taking its clothes. (After rescuing the fairy princess, her reaction was a Shout Out to Leia's reaction to Luke in A New Hope; in fact, that whole episode was loaded with Star Wars references.)
A frequent issue for Darkwing Duck; it seems Saint Canard thinks anyone in the purple suit, cape, and hat is Darkwing. Very annoying when your Evil Twin does this, of course.
Jonny Quest TOS episode "The Fraudulent Volcano". Dr. Quest and Race do this while escaping the enemy base.
The Powerpuff Girls episode "Bubble Boy" has Rowdyruff Boy Boomer captured by the girls. Bubbles impersonates him and infiltrates the Rowdyruffs in a bid to capture the other two. It's a nine-day wonder that Butch and Brick didn't catch on to Bubbles' noticeably higher voice and lighter eye color.
The Transformers has the episode "Masquerade" which features the Autobots replacing the captured Stunticons to uncover and sabotage Megatron's latest plan.
In nature, Batesian Mimicry is when a harmless species mimics a harmful one, for example, a moth larva looking like a snake head.
There are insects adapted to prey on ant larvae, which smell just like fellow colony-members to adult ants.
A number of German soldiers who dressed in American uniforms in late 1944 were summarily executed when caught. After all, as per the Hague Convention of 1907, dressing in the enemy's uniforms is considered a war crime.
The First Battle of Bull Run during the American Civil War had some Union soldiers in grey uniforms and Confederates in blue uniforms. Both sides had not adopted their standard uniform colors until a few months after the battle.
The battle at Pilot Knob ended with the union soldiers blowing up their cache of ammo and powder, and evacuating the fort under cover of night and rain wearing confederate uniforms.
It's not actually illegal per se to dress in the enemy's uniform while engaging in espionage or sabotage but it does mean that you are not a soldier in combat, which means you're an unlawful combatant, and the other side can do to you as they wish after they capture you.
It is, however, illegal to kill enemy soldiers while still wearing the enemy's uniform whether behind enemy lines or in the open battlefield as it constitutes unlawful acts of perfidy (opening you to war crimes charges).
This was the main modus operandi of the Selous Scouts, a Rhodesian special operations unit that would capture terrorists, turn them against their comrades (usually by giving them the choice between that and death) and then send them back to become The Mole. With them, of course, would be white Rhodesian officers or NCOs disguised as black terrorists.
Navies love this trope. During the Age of Sail, a popular tactic was to raise enemy colours (or, if you were really sneaky, a plague flag) and only lower them at the moment of firing. Died out after advanced naval gunnery and explosive shells meant that it was common practice to sink enemy vessels rather than capture them (the deception only worked if the disguised ship pretended to be a captured enemy one). Still the British raid on St Nazaire used this, flying a German ensign from the HMS Campbeltown. It worked.
A particularily funny example from World War One also combined with Because I'm Jonesy. The British and Germans both employed armed merchant ships (the Germans used them to attack British shipping, the British to hunt down German raiders), and often used fake markings, false smokestacks, and other elements to disguise them as the other side's ship. In the 1914 Battle of Trindade, the German SMS Cap Trafalgar encountered the British RMS Carmania, both of which were disguised. The problem was that not only was Cap Trafalgar disguised as Carmania, Carmania was also disguised as Cap Trafalgar. At that early point in the war, neither side actually knew which merchant ships the other had converted into auxiliary cruisers, and thus both ships' captains made poor choices of disguise.
Operation Greif: Otto Skorzeny and a small group of SS Commandos, fluent in English and wearing captured U.S. and British uniforms, infiltrated U.S. lines during the Battle of the Bulge with the main objective of capturing the bridges over the Meuse river. Although they failed due to an insufficient number of fluent English speakers, they did succeed in making the U.S. Army so paranoid about their presence and activities that many checkpoints were set up which slowed down the Allies.
In the end, an estimated 18 German commandos wearing U.S. uniforms were captured behind U.S. lines during the Battle of the Bulge, tried by military commissions, found guilty, and executed as spies.