Read or Die It's already been suggested that the British Library has a secret agenda, but the full extent of it isn't clear till they show up in force.
In the first episode of K, our hapless protagonist, Yashiro Isana, is being chased by people who want to kill him-and he has no idea why. He is saved by a total stranger, Kuroh Yatogami who just wants to kill Yashiro himself.
Ninja Scroll. Despite what he said earlier about the wisdom of remaining neutral, the lord of the Mochizuki clan turns up with his forces to help the protagonists take on the Devils of Kimon. But when Kagerou reports to her lord, he runs her through with his katana, as he's actually the Big Bad in disguise, and has been all along.
In Code Geass: Tales of an Alternate Shogunate, this happens to the villains; Suzaku and Euphemia's other forces have realized that Perry is trying to make Japan his own property, and as such, instead of helping him, fight alongside Lelouch and the Black Knights against him.
The 9th Crusade lead by Archbishop Maxwell towards the end of Hellsing is first greeted as saviors by the people of London against the forces of Millennium. Then they start indiscriminately mowing down everyone, human and vampire alike.
In Saint Beast, Goh, Shin, Rey and Gai turn against Luca and Judas at the moment they are most needed due to brainwashing, which isn't lifted until it's too late. Then, after Judas and Luca are banished to hell the others plot to free them with the help of the Goddess. The Goddess, however, is plain old sick of fighting and sells them out to Zeus. They get Taken for Granite and have their souls sealed away in darkness. And Judas and Luca are still stuck in hell.
Baka and Test gives us the "FFF". On the one hand, they'll help out in summon wars just as much as the next student in the class. On the other hand, if they so much as hear rumors that a girl's been even relatively nice to you...
In Thunderbolts, right after Baron Zemo revealed they were the Masters of Evil, most of the team revolted. They were getting their heads handed to them when the Avengers and the Fantastic Four broke in; one even expressed the view that she never thought she would be grateful to see them. Then Zemo revealed they were mind-controlled.
Usagi Yojimbo: Happens to Lord Mifume, which leads to his death, and to Lord Noriyuki's father.
In The Swarm of War, the Space Marines, twice. First, the ones woken up as a Superweapon Surprise turn out to be largely corrupted by Chaos. Later, during a battle between the King’s Army and the Chaos, forces led by the Loyalist Space Marines attack and wipe out the Chaos… and then the other side; due to the King refusing to recognize the Space Marines as his superiors, his kingdom is branded as heretics.
In Braveheart, William Wallace calls on his cavalry to help out the hard pressed infantry at the Battle of Falkirk... but Manipulative Bastard Longshanks had bought them off, thus securing an English victory.
There's a bit of a Double Standard in that this is considered a very vile act while early in the battle a large group of Irish from the English army outright switch sides and start fighting the English and it is treated as a great laugh. That battle has the English King being the Trope Namer for We Have Reserves, but still.
The dissonance comes from the fact that the Irish were stated outright to be conscripts (as in, forcefully recruited whether they wanted to fight or not). Conscripts in ANY era were notoriously unreliable, as they tended to either rebel or flee at the first opportunity. In contrast, the Scottish Cavalry were there of their own accord - if they didn't want to be there, they didn't even have to show up in the first place, Wallace certainly wasn't forcing them to be there.
In A Bridge Too Far This happens to the British paras at Arnhem as they hear approaching tanks and assume they are the British armored units that are coming to relieve them. It turns out it is a German armored division.
In Die Hard 2, the Special Forces unit called in to deal with the hostage situation turns out to be working with the terrorists.
Almost occurs in Jarhead when the Marines call in A-10 ground attack aircraft to provide support during a battle, only for the A-10's to mistake them for the enemy and nearly kill them all with the bombing run.
Judge Dredd. After a shuttle carrying prisoners to Aspen Penal colony crashes, a rescue/capture team is sent to locate Dredd, a convict on the flight. The Capture Team Leader reports to Judge Griffin.
Leader: There's no sign of Dredd. He appears to have survived the crash.
Griffin: You are in error, Capture Team. No one survived the shuttle wreck. Understand? Just find Dredd!
Leader: The pilot, sir. He's alive.
Griffin: No one survived the shuttle wreck! Do I make myself clear?
Leader: Yes, sir. [Shoots the pilot to death]
A similar case at the beginning of Tomorrow Never Dies, where the Big Bad has the survivors of the British frigate he sank machine-gunned while they paddled up to his ship hoping for rescue.
The end of the original Night of the Living Dead movie where the black survivor thinks he is rescued only to be shot by the redneck posse that mistakes him for a zombie.
In Red State, three local boys are taken prisoner by the Church to be sacrificed, and the ATF shows up outside to arrest them. When a local sheriff accidentally shoots one of the hostages, the ATF command orders everyone killed.
In Sharpes Battle, the seventh movie in a historical action series starring Sean Bean, this happens to the youngest member of the riflemen, a character who had been around since the first movie. Why couldn't they have used a red shirt?!
In Sin City, when the Federal Agents arrive at the farm, Lucille naturally assumes that they are there to help and yells at them to not arrest Marv, as he's with her. And then they pour boxes of bullets into poor Lucille.
An inversion in Star Trek: Nemesis. The Enterprise is barely holding its own against Shinzon's flagship, when suddenly they detect two more Romulan warships approaching. At first it looks like the situation's going From Bad to Worse, but it turns out that they're from a faction of the armed forces which remains loyal to the legitimate government that Shinzon's coup d'etat wiped out. They get their heads handed to them quite quickly, but the thought was there.
Another inversion in Taras Bulba (1962). The Cossack forces arrive in support of the Poles, who are losing in a battle against the Turkish forces. It turns out that the Poles were merely holding back so that they could treacherously attack the Cossacks after they won the battle for them.
When Dredd calls for backup at Peach Trees, Ma-Ma calls in support of her own in the form of some bought off Judges.
It happens a lot, most infamously at the end of A Game of Thrones when the Goldcloaks turn on Ned Stark.
And it happens twice in quick succession in A Clash of Kings, when Ser Rodrik's forces are betrayed by Ramsay Bolton's forces while they are about to launch an attack to retake Winterfell— and then mere pages later when Ramsay the Bastard of Bolton betrays Theon Greyjoy, the man who took Winterfell in the first place, after he opened the gates to his "allies."
During the Red Wedding, Roose Bolton and his men storm into the room. Catelyn thinks they're there to help her and Robb: however, Bolton is really in league with the Freys, cuts down his fellow northmen, and stabs Robb in the heart.
Lloyd Alexander's Chronicles of Prydain: In The High King, a powerful ally, who should be the reinforcements the good guys need against the army of the local Evil Overlord, turns out to be a traitor allied with said Overlord. Perhaps a mild subversion in that he pretty much rode into town and announced this a day before the big battle.
The Deluge (Polish Potop) when Prince Radziwil joins the Swedish, the Polish commanders who refused to join the treachery are imprisoned. There is a battle when their units try to break them out. Finally, the literal cavalry (the protagonist's unit) arrives... except he had unwittingly made a pledge on his soul to serve the prince, just in time to get himself stuck on the wrong side.
Inverted in J. R. R. Tolkien's The Lord of the Rings when the Black Fleet of Umbar, allies of Sauron, arrive at the Battle of the Pelennor Fields to the rejoicing of his armies. Only Aragorn had intercepted and captured it, and the ships are full of soldiers from southern Gondor.
In Graham McNeill's Warhammer 40,000Horus Heresy novel Fulgrim, the loyalist Marines on Isstvan V do not realize that only three of the Legions going to confront Horus are loyal; their other "allies" turn on them as recounted below.
Jack Chalker's Lilith: A Snake in the Grass features the sudden arrival of troops supposed to aid the witches in their attack on the protagonist's enemy, only for both defenders and cavalry to turn on the witches, the protagonist realizing he'd been an Unwitting Pawn in a plan to get rid of the witches.
In Star WarsLegacy of the Force, series villain Jacen Solo has his fleet surrounded by Confederation vessels in orbit of Kashyyk. He is just beginning to despair when the supposedly allied Hapan armada shows up. Thinking that victory is in his grasp he contacts the Hapan Queen Mother, his lover who is in command of the fleet, to coordinate with her only to have her give him an ultimatum to surrender.
Inverted during the Battle of Thull Mardu in The Belgariad, when King Drosta Lek Thunn of the Nadraks ends up clearing a path for the Western forces, betraying his fellow Angaraks, knowing that siding with the Mallorean forces would only mean subjugation, whereas siding with the West might get his fellow Angaraks' boots off of his neck.
Happens to the bad guys in the second Heroes Of Olympus book. The giant Polybotes is leading an army of monsters against New Rome, and he's pleased to see the Amazons (whom he had earlier tried to bring over to his side) coming to join the battle. Turns out they're here to help the Romans.
In Animorphs, this is a common role played by the Andalite military: The final book has the Andalites ready to destroy the human race rather than let it become a resource used by the conquering Yeerk Empire. Earlier books had other Andalites with plans was based on the same well-intentioned extremism: Arbat, who was probably just a rogue faction rather than representing the Andalites in general; and Alloran, who developed a biological weapon to eliminate the alien race of Hork-Bajir.
Live Action TV
In Season 4 of LOSTA boat arrives, that the castaways believe is going to rescue them, but in fact brings a team of mercenaries with orders to kill Ben and anyone who gets in their way. Some of them do get rescued anyway, but it doesn't help.
A similar thing happens at the end of season 1, with several characters getting on a raft and finding a boat which they think will rescue them, but instead it's the Others, who kidnap Walt and torch the raft.
The priceless Star Trek: Voyager episode "Message in a Bottle". The Prometheus is getting trashed by Romulans when some Starfleet ships arrive, only to attack everybody, since they know the ship had been taken over by Romulans earlier.
It doesn't help that the EMHs accidentally fire a torpedo at a Defiant-class ship.
This happens a lot in wrestling, though how often varies on how swerve-happy a given era and promotion is. A notable recent example was Matt Hardy running down to ringside with a chair during his brother Jeff's match with Edge at the 2009 Royal Rumble. Didn't take a lot to guess that Edge wasn't the one about to get smacked with the weapon.
Happened at the creation of the (in)famous Alliance, while Chris Jericho & Kane were being beaten down at the hands of the nefarious WCW crew. Then ran in for the save Tazz, Rhyno, Justin Credible (wait a minute)... Raven, The Dudley Boys... holy shit, Tommy Dreamer! Rob Van Dam! Jericho & Kane were promptly pronounced screwed when Paul Heyman left the announce table to proclaim that this invasion had been TAKEN! TO THE! EXTREEEEME!
WCW Bash at the Beach '96, which, in turn precipitated the biggest Face-Heel Turn in pro wrestling history. After Sting, Randy Savage, and Lex Luger had taken a beating from The Outsiders (Scott Hall and Kevin Nash), Hulk Hogan comes down to the ring, ostensibly to help out Sting and Savage (with Luger having been knocked out)...and then he leg dropped Randy Savage, revealing himself to be the Outsiders' mystery partner and heralding the beginning of the New World Order.
Generally, in wrestling, while heels will help each other, faces don't. If a face comes out to rescue another, particularly if he has no beef of his own with the heels in question, expect this trope to be engaged. It becomes more likely if the "savior" is carrying an object such as a chair.
Warhammer 40,000 turns this trope Up to Eleven. During the Horus Heresy, the Imperium sent seven legions to Isstvan V to crush Horus's insurrection. Three legions formed the first wave and met horrific resistance. When the other four landed, they expected relief and reinforcement. Out of hundreds of thousands of legionaries, only a few thousand escaped the Drop Site Massacre.
This also often happens when Inquisitorial forces show up to assist. They only tend to be called when the situation has gotten to the point where the only answers are 1) Kill It with Fire and 2) No Witnesses.
In Half-Life, the US Marines sent in to quell the alien invasion of Black Mesa are also there to contain all information of the outbreak by killing every Black Mesa employee they meet, including you.
Some aren't happy about it. Others are. Whatever it is, it gets justified quick when they think you're responsible for the whole mess and a psychopath.
The Komato in Iji combine this with Always a Bigger Fish. They're there to kill the Tasen, but you're...well, there.
In World of Warcraft, the battle of Angrathar contains a genuine example of The Cavalry when Horde wolf-riders charge into battle to help the beleagured Alliance. But when the Lich King himself emerges, and Putress bombards the entire battleground with Blight.
The famous Wham Level of Modern Warfare 2 ends with one of the most brutal cavalry betrayals in fiction: The team holds a cabin against an endless stream of attackers while a huge amount of data is copied on a portable hard drive and most of them are dead when all of the data is secured. The last men make a frantic dash down a hillside without any real cover while under fire from lots of automatic rifles, machine guns, and rocket launchers and just 14 meters from the extraction point the player gets downed. When you regain consciousness a few seconds later, your last teammate Ghost drags you by your jacket and finally helicopters of Shadow Company appear from behind you and mow everything down with their miniguns. And when you finally stumble the last steps to the evac chopper, General Shepherd takes the hard drive and shoots both you and Ghost in the chest. The last thing you see is your bodies being thrown in a ditch, a soldiers emptying a canister of gas over you, and Shepherd throwing his burning cigar at your face. You really didn't see THAT coming.
Likewise Fire Emblem Genealogy Of Holy War, although the lead-up makes it a bit easier to see it coming. Chapter 5 looks like it's going to be the final battle against all of the antagonists, but rather unexpectedly Velthomer switches sides and starts attacking Freege's army as soon as you get close to Velthomer, double-crossing its allies. All this in spite of the fact that Arvis has now been revealed to be in league with the Loptyr Sect. The chapter ends with Sigurd being led to Belhalla, ostensibly to be cleared of all charges, only to learn that he is still labeled as a traitor and will be killed, along with much of the party (though their fates are mostly left ambiguous).
Its midquel Thracia 776 features a sort-of Meta example: late into a grueling defence chapter, an army of green (NPC) Thracian Dracoknights arrive and start heading for the city. The player will probably be exclaiming "I'm saved!" at this point, as the Non Player Characters fly in and clear out the enemies. Until they actually reach the city, upon which they start attacking you. Turns out, they're on neither side: they just want the city back under their control, meaning you're just as much of an obstacle to them as the enemy army. Notable in that it's possibly the only time in the entire series where green NPC units can actually attack you.
There's a Japanese DOS-era tactical RPG called Legend of the Seven Heroes 2 that ends like this. After a Climax Boss battle against the enemy nation's Big Bad General and his army, an army from the friendly nation shows up, and the main character assumes they're there to rescue them. However, the friendly nation's General arranges a truce with the Big Bad General and decides to kill the main characters, because he figures that the main characters' survival would bring peace between the two nations and thus be politically disadvantageous to both of them. The final battle of the game thus has your 7 characters being attacked from the North of the battlefield by the Big Bad General and his army and from the South of the battlefield by the not-so-friendly nation's General and his army.
Towards the end of 'Dragon Age: Origins: Awakeningyou have the option of doing this to the City of Amaranthine. The city has been overwhelmed by darkspawn while Vigil's Keep has been ambushed by an even larger darkspawn army. You have the option to burn the city down with Arrows on Fire, killing all the people and darkspawn that your soldiers have trapped inside, freeing you to save your fortress. Fortunately, you can Take a Third Option and save everyone if you have built up the fort enough that it can stand without you.
This happened in the Backstory of one of the towns in Dragon Quest VII. When the monsters first appeared and started terrorizing the town and kidnapping people, one man convinced his neighbors to stand against them. He went ahead to start the fight... and everyone else chickened out. To twist the knife further, this betrayal caused his sister Matilda to hate everyone, allowing the monsters to turn her into a Tragic MonsterBarrier Maiden.
An extremely rare villain version happens in Assassin's Creed: Brotherhood, during Sequence 8, where after destroying all of Cesare Borgia's support, he sees one last chance in his Papal Guard army arriving at Roma's gates, only for them to arrest him on orders of the new Pope.
After the finale of Left 4 Dead, the characters are rescued by the military in an APC. The subsequent comics show that the army locked them up out of fear that they were carriers and intended to kill them before a horde rush allows them to escape.
In Ys Seven, after getting the last crest from the Sea Dragon on Ruin Island you go back to your ship only to be attacked by a giant Titano, Just in the nick of time the Dragon Knights show up and Save you. That is only half true however, They are actually here to rescue Aisha and Arrest you for "kidnapping" her and assassinating the King who was murdered mere moments after Adol leaves said King's bedroom.
The player character pulls this off in the endgame of Fallout: New Vegas if they're fighting for Mr. House or an Independent Vegas. While you and the Securitrons are fighting alongside the NCR for the majority of the battle, you're also secretly trying to activate the Sealed Army in a Can. Once the Legion is routed, you then have to get General Oliver to retreat, either through force or through diplomacy.
In The Gamer's Alliance, a regiment of Proninists disguised as Scun Loyalists manage to fool the Maar Sulais army into letting them get closer during the Second Battle of Victoire. They reveal their true colours by attacking the surprised Maar Sulais defenders who then are forced to flee as they're being outnumbered.
In Worm, Echidna has stalemated Eidolon and her clones are whittling down the combined forces of the Protectorate and the Undersiders. Then the Travelers, the Undersider's allies, show up to help, but Trickster, the leader of of the Travelers, betrays everybody in favor of his ex-girlfriend.
Used in a Cliff Hanger in He-Man and the Masters of the Universe (2002): Adam, deprived of his sword and stuck facing down a group of villains led by Skeletor, thinks he sees the Masters on the horizon. They turn out to be Skeletor's minions using the Masters' vehicles.
This was part of Megabyte'sgrand strategy during the Web World War: he waited for Mainframe's CPU fleet to take heavy casualties before deploying his own forces, and the moment the invasion ended, he immediately directed his troops to destroy the city's remaining defenses. It was this betrayal that ultimately allowed Megabyte to conquer Mainframe.
The Soviet Union did this when the Polish Underground launched Warsaw Uprising against the occupying Nazis in 1944.
Stalin LOVED this trope, as he encouraged the Pro-Western factions in Eastern Europe to rise up when the Red Army advanced, let them bleed themselves out against the Germans until they were destroyed, then kicked the Germans out and set up his own puppets.
In an inversion of the above, Hitler also played this role in the areas of Europe the Soviets occupied prior to 1941. Indeed, many guerrilla organization (such as the Forest Brothers and the UPA) saw him as a potential savior, only to find themselves surrounded by Feldgrau and Panzers gunning for them (though Hitler did allow some anti-Soviet fighters to join his puppet organizations).
The Battle of Bosworths Field. The Stanleys, nominally allied with King Richard III, waited until he was separated from his army and charged the King's Bodyguard in the rear.
This was how the Bosnian uprising under Husein-beg Gradaščevićes leadership against the Ottoman Empire was squashed in 1850. After routing the imperial army for more than a decade, the final battle that would decide the fate of Bosnia until this day was being fought in what is today the Stup neighborhood in Sarajevo. After defeating the Ottomans, the exhausted Bosnian rebels were overrun by what were supposed to be their reinforcements from Herzegovina- who thought the country would be better off in the crumbling Ottoman Empire.
The French did this to their Genovese allies at the Battle of Crecy. The Genovese crossbowmen, forced by their French paymasters to fight without their pavises (huge shields that protected them from the murderous English longbows as they reloaded), were massacred by the English. When they began to retreat, the French knights charged right over them, either because their commanders were angered by the "cowardice" of the Genovese or were simply too impatient to wait for the exhausted Genovese to withdraw. The French knights paid for their arrogance and bad tactics, however, as they were also crushed by the English forces, faring little better than the crossbowmen had.