Crime is rampant on the streets. Drugs are being sold, people are dropping at the sound of two boards being slapped together, windows are shattering to make way for burglars... who knows what sort of craziness goes on every day? Then, police officers show up to either take back the loot or perform a vigilante killing, and it looks like the day is saved...
Not! Surprise, surprise; sometimes by way of a Traitor Shot, the police officers are revealed to be scofflaws themselves (or, at the very least, a bunch of people who have taken the law into their own hands). Naturally, this upsets the balance of law and order between criminals and the police; Hilarity Ensues (as does chaos every so often). At times, it's a single person acting alone; at others, it's an organized group; but in fact, impersonation of a police officer has taken place.
Related to False Flag Operation or Dressing as the Enemy. Compare Bavarian Fire Drill, a.k.a. getting people to do what you want by acting like an authority. Unrelated to No Badge? No Problem!, which is where someone affiliated with the authorities but who actually isn't one acts as though he were.
Anime and Manga
Lupin III: This happens to be one of Lupin's favorite tactics, often by disguising himself as Inspector Zenigata; usually at Zenigata's expense.
In The Castle of Cagliostro, Lupin infiltrates the titular castle by posing as Zenigata, claiming that the real one Gustav saw was an imposter. It works. Gustav falls for it and attacks Zenigata and his men, allowing Lupin to slip inside unnoticed.
The first happens, near the beginning, where he disguises himself as one of Zenigata's men in an attempt to slip past the inspector (which Zenigata doesn't fall for).
The other happens about halfway through the film, when he disguises himself as a police officer to infiltrate Morocco's police HQ to dig up information on Galoux. Which is leads to a run-in with Fujiko and a night of Sex With The Ex.
Lupin poses as Zenigata again, in the Red Jacket episode "Albatross: Wings of Death", where he uses the disguise to try to get Prof. Lumbach to tell him about his bomb manufacturing plant. Lumbach stalls by pretending to fall for it, to buy time for the real Zenigata to show up!
And he poses as the inspector one last time, in the Red Jacket series finale "Aloha Lupin", to track down a group of imposters who were impersonating him and his gang.
The debut episode for Sailor Mars includes a heroic example. When Bunny Tsukino finds out that the disappearing buses had been hijacked by Dark Kingdom agents, she boards one disguised as a cop and tries to arrest the driver. It backfires on her as she ends up being forced into another dimension along with the bus.
Three episodes later, this gets subverted when it turns out the security guards attacking the Sailor Soldiers are not only fake cops, they're also fake humans! That's right, Jedite created them from scratch and sent them to attack the Sailor Soldiers as part of a backup Batman Gambit in case they ended up defeating him. Unfortunately for him, after his defeat, an impatient Queen Beryl committed an act of Bond Villain Stupidity, and he became just another case of Death By Secret Identity.
The Punisher has been known to use fake ID to enter crime scenes and get firsthand information before the detectives arrive.
An early comedic example of this ploy in play is The Wrong Arm Of The Law, where a gang of Australian crooks upsets the established rules of the cops and robbers game in London. Shortly after this gets taken too far when a couple of paranoid lower-level criminals unwittingly attack actual police officers, the criminal underworld decides to forge a temporary alliance with the police to capture the IPO mob.
In The Streetfighter's Last Revenge, shortly after being double-crossed by the Owada clan, Terry Sugury intercepts a group while disguised as a highway patrol officer. He directs them to pull over in a car crushing lot and then reveals himself. He then sends a lone assassin, Wolf, back to the Owada clan alive to tell the patriarch that he should fight Sugury for the money he had stolen.
As seen in a flashback, Chow pulling one off sets the plot of The Hangover Part III in motion.
Inspector Gadget, meet your Evil Twin Robo-Gadget, who's not so much impersonating a police officer as he is downright smearing the police officer's good name by committing all sorts of crimes.
Terminator 2: Judgment Day: Clearly the favorite form for the T-1000 Terminator to take is that of a Los Angeles patrolman. Not surprisingly, an LAPD cop was the first person the T-1000 encountered upon arrival from its time-travel. That form also avails the T-1000 to plentiful information and resources.
Dick Tracy. Several of Big Boy Caprice's henchmen dress up as police officers, "arrest" Lips Manlis and take him to a warehouse to be murdered.
As Ra's al Ghul claims in Batman Begins, Bruce Wayne is trying to defend "a city so corrupt [the League of Shadows] infiltrated every level of its infrastructure. Effortlessly." How effortlessly? The district attorney, Carl Finch, was assassinated by what appeared to be a police officer.
In The Dark Knight, this is part of The Joker's assassination plot against Mayor Garcia at Commissioner Loeb's funeral. Fortunately, Jim Gordon figured out that something was up just by reading an obituary the Joker had typed up for Garcia, came to the funeral wearing a bulletproof vest, and dove in front of the Mayor just as the Joker and his goons fired at him.
When we first meet Harry in Home Alone, he poses as a cop, check in on all the houses in the neighborhood, seeing who will be leaving for the holidays, allow him and Marv to rob them later.
In Running Scared, the drug dealer Julio Gonzalez has his men capture the police officers guarding a government building and put on their uniforms to masquerade as them. He then uses the building as the location to trade a woman he captured for the cocaine the protagonists seized.
Ostap Bender tries to do this with the underground millionaire Koreiko in The Little Golden Calf. Ostap wants Koreiko to admit that a large sum of money was indeed stolen from him, to confirm that he's richer than he seems. However, it's a Paper-Thin Disguise consisting of only a police hat, and the hat has a coat of arms of the wrong city, to boot. Koreiko later points that out.
The Three Investigators has a Classy Cat-Burglar have one of his men dress up as a police officer. When the cops show up and try to use that as a charge, he points out the the fake cop is in fact wearing a New York Police uniform (the series is set in California), and as such cannot be accused of impersonating the local police.
Seven Days: Two inner city hoods steal Frank's and Olga's NSA IDs, and are later seen at a bar demanding that the bartender give them a bottle of some expensive alcohol, but the bartender isn't buying it. "OK, if you're with the NSA, what does 'NSA' stand for?"
My Name Is Earl: Earl stole a cop's badge and used it to get free food and other stuff. Then someone stole it from him.
In Supernatural Sam and Dean regularly go undercover as FBI agents (as do several other hunters), with Bobby backstopping their aliases if someone wants to call their superior. One episode shows Bobby has a whole wall of phones labeled with each alias, though in the same episode the trope fails because it turns out the sheriff they're talking to knows Bobby.
Stupid crook Dewey Crowe decides to rob two toughs who stole a large amount of drugs from the Dixie Mafia. When the local clothing store does not have any ski masks in stock, he instead buys a suit and a cowboy hat. He then proceeds to impersonate US Marshal Raylan Givens and successfully pulls off the robbery. When the real US Marshal Raylan Givens finds out about it, he is quite pissed and tracks Dewey down. It does not help matters that the two toughs have also tracked Dewey down and when Raylan identifies himself, they open fire on him since they will not be fooled by the same trick twice.
In season four, a Detroit hitman is tasked with killing Drew Thompson but no one knows what identity Drew is currently using. The hitman disguises himself as a sheriff's deputy and goes to the houses of men who could be Drew and shoots them dead. He is exposed when he tries to arrest Boyd Crowder and Raylan Givens is present. Raylan just spoke to the sheriff and the sheriff would have mentioned if he sent someone to arrest Boyd. Before Raylan can check in with the sheriff, the hitman panics, draws his gun and is shot dead by Raylan.
In an episode of Adam-12 there's someone out there claiming to be a particular detective in the LAPD, flashing his badge around and inisiting on bribes. They aren't sure until the end of the episode whether it's really that cop turned bad or someone impersonating him. It's someone impersonating him.
Alluded to a few times on It Takes a Thief (2005). Some of the homeowners who signed up for the show were either police officers or married to police officers. Matt always made a point of talking about what would have happened if a real burglar, instead of Jon, had walked off with police uniforms, badges, and firearms.
In an episode of The Listener a gang of robbers use fake cop uniforms to gain access to secure locations like high end jewelery stores and rob the place. Their disguises are very good and they even make sure that their getaway car looks like an authentic police vehicle. Later in the episode they steal an ambulance and commit a robbery disguised as paramedics.
In an episode of Barney Miller, a man claiming to be a detective from the 12th precinct is accosting men as they leave gay bars and demanding money or else he'll beat them.
Orphan Black has petty criminal Sarah witness the suicide of a woman who looks exactly like her - who ends up being a seemingly loaded police detective. Wanting the money for her daughter, Sarah impersonates her and attempts to ingratiate herself into her life - it works, for the most part, until the other cops catch on. Of course, since she and the woman were clones, it's not like Sarah had a particularly hard time of it.
In Doctor Who we have the Doctor's psychic paper, a business card of sorts that appears to the reader as whatever form of ID he requires at the time. He's done everything from a simple Bavarian Fire Drill to more involved impersonations of plainclothes law and military officers.
In Mission: Impossible members of the IM Force often impersonated police officers...as well as soldiers, security personnel and other officials.
Banshee combines this with Dead Person Impersonation. Lucas Hood was the sheriff of a backswoods town in Oregon who decides to take the job offer of sheriff in Banshee, Pennsylvania. However, on his way into town he stops at a bar on the outskirts of Banshee and is killed by two robbers. The robbers are then killed by the protagonist, a master thief who just got out of prison and is in Banshee looking for his old girlfriend. The thief realizes that the only person in Banshee who actually knew what Hood looked like recently died of cancer, so he assumes Hood's identity and becomes the new sheriff. He has no police training but his brutal methods are quite effective against the local toughs so people just assume that he is a Cowboy Cop.
John Reese has repeatedly used the badge of Detective Stills, a Dirty Cop he killed in the first episode. He also once used the ID of Jennings, a wife-beating U.S. Marshal he deposited in a Mexican prison.
Carl Elias' number two, Anthony "Scarface" Marconi, was first seen impersonating a patrol officer.
One episode had a pair of ex-FBI agents who had been forced out for corruption, but were now pretending to be FBI while acting as hitmen.
Shadowrun supplement Bug City. Truman Technologies operatives dressed up as Eagle Security officers captured Fuchi-employed gang members, lined them up against a wall and ruthlessly murdered them. This was inspired by the 1929 "Saint Valentine's Day Massacre".
One episode of Batman Beyond shows people from the criminal organization KOBRA dressing up as cops to get Batman to willingly hand over a boy they've been targeting, who knows what Batman looks like under his mask.
Cartman's brief impersonation of a police officer in an early episode of South Park. He dressed like a police officer (complete with aviator sunglasses) and pulled people over in his big wheel. When they inevitably figured out that he wasn't a real cop, he'd start beating them with a baton.
"RESPECT MY AUTHORIT-AH!"
The 1929 "Saint Valentine's Day Massacre". Killers hired by Al Capone's gang dressed as police officers, captured members of the North Side gang and executed them.
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