Happened more than once in Sailor Moon. This was lampshaded in the Live-Action Adaptation, where Queen Beryl explains to an annoyed minion that she's aware her generals don't always particularly like her, so she allows the in-fighting between them as a distraction that pushes them to be more creative against each other.
Subverted in Soukou No Strain: the traitor openly plots against Ralph for ages, but gets killed off before he can do anything. He simply wasn't a threat.
Thorfinn in Vinland Saga makes no secret of his homicidal hatred of Askeladd. Askeladd is still able to sucessfully use him for any job he needs as long as he promises to duel him in return.
Motoharu in To Aru Majutsu no Index is working for several organizations as a spy... and every one of them knows it. However, since he's pretty good at what he does, they tend to overlook it.
Ichimaru Gin of Bleach. He tells you straight to his face that he's a snake.
Aizen was actually counting on this, Magnificent Bastard that he is. He figured that Gin would betray him at some point (though he didn't realize the real reason why Gin would do it), and even acknowledged there was a chance that Gin would successfully kill him in the betrayal. But he needed a genuine risk of dying to kickstart his "evolution".
Aquarius from Fairy Tail will attack everyone indiscriminately, friend and foe alike. But when all's said and done, she usually ends up being helpful overall.
The X-Men franchise has Sabretooth. Whether it's Weapon X, the Brotherhood, Apocalypse, or even the X-Men themselves, no one who employs him ever really trusts him. With good reason, too, as he's one of the Ax-Craziest of Marvel's creations.
V for Vendetta has Inspector Finch. He criticizes Susan's dictatorial regime to his face. As Sutler's puts it, the fact that Finch is still alive proves how appreciated his work is.
Mycroft Holmes invokes this with regards to Campion Bond in The League of Extraordinary Gentlemen. When Mina asks if Bond will be punished for his deeds, Holmes replies in the negative, saying, "It is often useful to have employees whom you know to be treacherous."
In Sonic the Hedgehog, Eggman has stated that he knows full well that Snivley, Lien-Da, and several others are planning to overthrow him, but because of that they will work the hardest for him, because they want to inherit a strong empire. He actually goes through the trouble of saving Lien-Da simply because he was impressed by her attempt at his position.
It was revealed that Loki allowed himself to die at the end of "Siege" and be reborn as a child because he realized he had become a Reliable Traitor. As a god of trickery and chaos, he could not bear to be predictable. In the end, he even betrayed himself.
The One Sith in Star Wars: Legacy try very hard to avoid being Reliable Traitors. The self-serving nature of the Dark Side makes it very difficult. All things considered, the do a better job of it than most Sith orders that don't adhere to the Rule of Two.
The Joker, flat out. It's not so much a question of "if" he's going to turn against your merry little band of super-villains, but when. Best case scenario he'll just get distracted at an inopportune time and wander off. So why bother? As pointed out by Lex Luthor, the alternative is to not let the most unpredictable and dangerous villain in the DC Universe have his fun, which just begs for trouble of a different kind.
In Les Légendaires, Tenebris warns her father Darkhell about Raptor's obvious ambition and Starscream tendencies. Darkhell basically answers he is aware of it, but still keeps Raptor at his service regardless because he think such ambition only increases his motivation to succeed in his mission.
Capt. Renault in Casablanca is this. He openly admits that he "blow[s] with the wind", and the wind is blowing from Vichy, so he collaborates with the Germans. The film ends with his memorable Heel-Face Turn.
Any Sith in Star Wars, given that Chronic Backstabbing Disorder is one of the side-effects of The Dark Side. The Star Wars Expanded Universe is filled with examples of characters using this against the Sith or a previously powerful Sith Empire eating itself because all of its members were... less than trustworthy. The Rule of Two - which is how the Sith operate by the time of the films - was intended to control this treachery and actually turn it into a strength, though given how both Palpatine and Vader fully expected the other to use Luke against them it wasn't a perfect arrangement.
Jack Sparrow of Pirates of the Caribbean has betrayed every other character in the series at least once, but he's so motivated by self-interest (usually) that he can still find allies.
In Superman II, Superman's plan to defeat General Zod hinges completely on Lex Luthor selling him out after learning how he can be depowered. He had earlier altered the device so that the only safe place was inside it.
The heroic version is exemplified by Major Elim Rawne of Gaunt's Ghosts. Early in the series he doesn't disguise the fact that he hates Colonel-Commissar Ibram Gaunt and, when the time is right, fully intends to kill him. He even tells people this. Gaunt not only knows, he gave him the knife (to replace one Rawne lost) which everyone expects Rawne to use in the attempt. He eventually mellows out about the whole "Killing my commanding officer" thing.
David Eddings' Elenium and Tamuli series, Krager is regularly kept around by both sides, the villains because he's a competent minion, the heroes because he coughs up information worth a lot more than his life fairly willingly. Ironically, at the end of the Tamuli series, he's dying of self-inflicted illness due to his heavy alcoholism.
Lady Invidia Aquitaine has betrayed almost every character in the series at least once, to the point of being The Dragon to two Big Bads (both the initial apparent one and the ultimate real deal) while having several Enemy Mine moments with the heroes. Ultimately, though she shows some signs of a Heel-Face Turn as she gets increasingly disturbed by her current inhuman boss, she ends up solidly a villain. Eventually the Vord Queen realizes just how reliable she is by predicting her latest betrayal and turning it to her advantage. And isn't in the slightest annoyed by it.
On the other end of the spectrum, we have Fidelias, who fits the description above almost perfectly. He turns on Gaius Sextus because he feels his politicking is destroying the country, and becomes the Aquitaines' chief spy despite not liking them much because he thinks Attis would be the best leader. He remains absolutely reliable to them until he decides Gaius Octavian would make a better First Lord, at which point he delivers his letter of resignation with a poisoned balest bolt.
Also from Butcher, The Dresden Files has this relationship among many of the darker powers. As of Ghost Story, Harry himself has promised to become Mab's strongest and most reliable Winter Knight, but without truly being her ally.
The non-subordinate version can be found with the Denarians, who, without question or exception, break their word the very second they think they can get away with it. In many cases this comes across as short-sighted Stupid Evil; most of the villains of the series always keep a promise or bargain (either from personal honour or because it makes diplomacy and negotiation much easier) and can thus negotiate with the other powers, while the Denarians have to operate alone because everyone knows they can't be trusted, and most of the betrayals get defeated pretty quickly because they were predicted. Nicodemus, however, is sufficiently clever that he still manages to surprise people with the manner of his betrayal, even though the betrayal itself is foreseen a mile away, and can force people to deal with him even knowing he will stab them in the back when he can.
Colonel Thomas Blood in The Pyrates. He even warns the hero of this, when he first signs on. Later, when Blood runs out on the hero, he leaves a note which actually says:
Ye mind I warned you I might play false. Well, this is it, ould joy, and if I said it grieved me I'd be a liar, which I am, but that's naught to the matter.
Diana from Gone is very open about the fact that she can't be trusted.
Caine: Whose side are you on, Diana?
Diana: I'm on my side.
Narses in Belisarius Series is an interesting example. He betrayed conspired with agents from the evil Malwan empire against the Emperor and Empress of Rome. He intended however to betray Malwa itself all the time which he does eventually just in a different manner. He is a skilled chessmaster, and his chief rival, Irene points out that Narses' loyalty isn't to be trusted but his skill in chessmastery is.
Denth in Warbreaker is a friendly mercenary who likes to joke about how as a mercenary he has no loyalty other than to whoever pays him most. As with Littlefinger, he openly reveals his treachery to his future victims, because he realizes that being Wrong Genre Savvy, they interpret his openness about his treacherousness to mean that he is trustworthy.
Tama Montezuma, mistress of racialist leader Marengo North English from the Garrett, P.I. novels, doesn't make any pretense that she's not staying with him because he provides her with a lavish lifestyle. Her honesty about regarding him as a sugar daddy blinds him, and other men she's seduced the same way, to the fact that she's been putting words in his mouth to exploit his underlings and allies while ostensibly acting as his intermediary.
Live Action TV
All over the place in Hustle. A lot of the time, their marks are so reliably likely to turn on the gang that Plan A relies on it entirely. In some cases, it even goes so far as to mess up a perfectly good plan when the don't try to betray the gang...
Subverted with Jayne. Everyone fully expects him to turn on them if given enough (monetary) incentive and are prepared for it, however not only is Jayne far more loyal than even he realizes but the time he does betray them isn't suspected at all, and Mal is far more furious than he would be if he expected it.
Saffron is a straight examples. When she comes to the crew to get them to take part in a heist, they immediately suspect she will turn on them. They create an elaborate ruse to play off her inevitable betrayal and take her for the fool in the end.
Supernatural has one of the Winchesters point out that the Trickster/Gabriel is playing a trick on them. His response? "Hello, Trickster?"
The demon Crowley starts out like this, before heading sharply into Magnificent Bastard territory.
Classy Cat-BurglarBelaTalbot may also count. She spends half of her episodes as a sort ofFriendly Enemy to the Winchesters before cementing her position as an antagonist by stealing the Colt and trying to kill them in her desperation to get out of her own Deal with the Devil. In her final moments alive, she tattles on Lilith to the boys because Bela wants her dead for using her.
The Leverage team can only beat Sterling when Sterling wins too.
Sterling: Your entire plan relied on me being an arrogant, self-serving bastard.
Nate: Oh, yeah. That's a stretch.
On Angel Harmony can be relied on to apologetically backstab her friends whenever a better option presents itself, though she doesn't like to admit it. Her backstabbing them actually factors into their plan in the finale.
Angel: Loyalty really isn't high on your list.
Harmony: Oh, is that right? I'll have you know I am damn loyal, dumbass.
Angel: You betrayed me. You are betraying me now, even as we are talking!
A few castaways throughout the seasons of Survivor have fit the bill, but the most straightforward example is Cook Island's Jonathan Penner. After mutinying from the Aitu Tribe when the opportunity presented itself, he later turned on his new alliance to help the tribe he left behind conquer a numbers disadvantage during the Tribal Merge. The reason? Yul Kwon, (Aitu's de facto leader) threatened Jonathan with his hidden immunity idol, knowing that Jonathans' interest in self-preservation was greater than his loyalty to the Raro Tribe.
For the first couple seasons of Farscape, Moya's crew could always count on Rygel to abandon and/or betray them whenever things got tough. He got better...eventually.
On Stargate Atlantis you can always count on Todd to turn to join forces with Atlantis for an Enemy Mine situation or run to them for help. You can then count on him to betray them as soon as he possibly can.
Mitchell of Being Human describes himself this way, neatly summarizing his various changes of allegiance by saying that he will do anything to survive. He explains to Herrick, "Right now, the only way for me to stay alive is by siding with you. Completely and 100%? You bet. Forever? I doubt it."
Kharn the Betrayer. Almost everyone knows that he'll kill anything near him just for being near him, enemy or not. It's a testament to Kharn's bloodlust (and his compatriots' insanity/stupidity) that this doesn't usually save them.
In Exalted, this is the very core of Ebon Dragon's being: you can count on him betraying you at some point. In fact, he can count in him betraying himself at some point.
"Darkness" (it's aCode Name) from BIONICLE hangs over the Shadowed One's shoulder, waiting for him to show weakness so he can kill him and assume leadership of the Dark Hunters... The Shadowed One is well aware of this and actually encourages it; the idea being that this way the Shadowed One will be ever vigilant and never go soft.
In the Barbarossa campaign of Age of Empires II, Henry the Lion betrays Barbarossa, but is still forgiven, considered a valuable ally. Then he betrays you again, and is banished.
Etna from Disgaea makes it rather clear to Laharl that she'll kill him where he stands if he doesn't prove himself worthy. Laharl just sees this as even more of a reason to promote her to second-in-command. She did take out a key enemy, after all, so she gets the job done.
Archer in Fate/stay night. Lancer and Assassin also have shades of this trope, being motivated by being Blood Knights while being stuck with Masters who deny them battles, but neither of their masters appreciate them either.
Winter Assault features two very brief alliances, which are brief because each side expects the other to betray them, and each side doesn't disappoint. However the fact that each of them betrayed the other side because they expected the other side to betray them might put this into Self-Fulfilling Prophecy territory.
In Dawn of War II: Retribution, Abaddon fully expects Eliphas to betray him at the first opportunity, but isn't concerned as for the moment Eliphas is completely under his power and must follow his commands. This incredibly short-sighted behavior might contribute to Abaddon also being known asFailbaddon.
Warcraft has plenty of these. First case was Evil Sorcerer Gul'dan, who Orc Warchief Orgim Doomhammer kept around because he knew Gul'dan would be useful to the Horde, even though he didn't trust the warlock. Statements about Gul-dan say that he preferred having himself surrounded by untrustworthy, ambitious individuals like himself because he knew how they thought, and actually didn't like having individuals motivated by loyalty around him because he had no clue how they thought. Additionally, Ner'zhul, the Lich King, works for Kil'Jaeden despite quite clearly having ulterior motives—since Kil'Jaeden never trusted Ner'zhul, he had his Elite Mooks, the Dreadlords, watch the Lich King.
Some early editions of Romance of the Three Kingdoms on the SNES had an oddly literal example in Lu Bu. While undeniably the deadliest general in the game whose WAR rating was unparalleled, he was almost impossible to use in that function because he could be counted on to turn on whoever he served if you provided him with a modest bribe. Even measures to ensure high loyalty wouldn't come to much—a little more gold and your best warrior was off to join the opposition. On the other hand, coming across Lu Bu was not as threatening as it might be in other games, because any half-decent strategist could bribe the clueless brute in return, bringing him over to your side for the turn. You could even mess with history by doing this to Dong Zhuo during the campaign to overcome him. It eventually leads to the bizarre situation where you have the greatest warrior in the land, and you're keeping him holed up in some corner of the kingdom training troops for the rest of your officers. While the early games didn't offer the greatest amount of characterization, this is Lu Bu we're talking about. I think it's been demonstrated that he's not exactly the most loyal of people.
In Arc the Lad: Twilight of the Spirits, Darc (a deimos who believes firmly in Asskicking Equals Authority) spurns letting any deimos who is slavishly loyal to him serve him, since he sees it as a sign that they're weak and would turn on him as soon as a stronger overlord turned up. In contrast, he happily welcomes Delma back into his service after she stabs him in the back, leaves him for dead and tries to steal control of his clan from him, seeing it as an acceptable display of strength on her part.
Knights of the Old Republic, being part of Star Wars, not only inevitably features this amongst the Sith but even allows the player to implement it in the Sith Academy, playing Reliable Traitors against themselves as a distraction for their own treachery.
In Dragon Age: Origins, the Tevinter magister Caladrius is openly contemptuous of his patron, Teyrn Loghain, and immediately offers to betray him to the protagonist if he is allowed to live and still make a profit on his Evil Plan.
In Tales of the Abyss, Dist the Reaper of the Six God-Generals makes no secret that he's only there so Van will provide him the materials to do his fomicry research. His one and only goal is bringing his dead mentor back to life (thus forcing Jade to acknowledge his ability), so as long as they're a useful means to that end, they can count on him.
Captain Scarlett of the first Borderlands 2 DLC is completely honest about her treacherous nature (her intro even states "Will Absolutely Stab You in the Back") but states that she'd much rather work with you to find the treasure before backstabbing you. She is completely honest in this regard, and does stab you in the back as soon as you discover the location of the treasure.
Saemon Havarion of Baldur's Gate always betrays someone whenever he appears; interestingly, the betrayed is only sometimes the Player Character. He also proves to be much more trustworthy when sufficiently motivated (i.e., after all of his friends and associates have been betrayed and murdered).
In the Sluggy Freelance storyline "Oceans Unmoving," Bun-Bun purposely makes the most untrustworthy member of his pirate crew the first mate, because he "feels safer knowing where the next mutiny will come from."
In Girl Genius, the entire Sturmvoraus clan seems to breed nothing but these.
In Whateley, half of the campus security is made of these. EVERYONE is spying on EVERYONE for SOMEBODY, but it all cancels out, so everyone ends up working as reliable cops. Some are more snarky and bickering then others, though.
There was that one guy with Humans First, who tried to kill Tennyo... Ayla successfully manipulated him into this trope.
In Transformers: Beast Wars, Megatron thinks of Tarantulas this way, who presents a facade of loyalty and fools Megatron about as often as Megatron sees through it.
From what impressions we are generally given of Predacon 'culture', betrayal and treachery are expected, as they ensure that only the strongest and most capable remain in command. "I can tolerate your treachery, lieutenant, but NOT YOUR INCOMPETENCE! Treachery requires no mistakes."
The other spider, Blackarachnia, fits into this trope much more neatly, often voicing her dissatisfactions. Notably, her ambitious personality was purposely designed by Tarantulas, whose Mad Scientist expertise first converted her from a Maximal. Turns out she was pretty much like that anyway even without the cerebro shell, which makes you wonder what the point was.
Yet another example is Rampage, who openly has no loyalty to Megatron, but is kept in line by routine reminders of the Restraining Bolt and the upside of opportunities to terrorize and demolish.
On the subject of Transformers, this was the exact reason why the Trope Namer for The Starscream was not destroyed. He was a capable scientist and the leader of the best fighters in Megatron's army and therefore inexpendable. This has been remedied in Transformers Animated, where he does not have such luck and is promptly killed the moment Megatron gets his hands on him. he came back a lot. Five times in one episode of Animated. Death Is Cheap (for Starscream at any rate).
Ironically, this occurred to the original Starscream as well, who came back as a ghost.
Granted, Prime also demonstrates that Megs is probably well aware his entire crew except for Soundwave would off him if they thought they could get away with it. Starscream's just the only traitor with the guts to be proactive about it, which kept Megatron entertained until Starscream started becoming too predictable to be of any use to him.
Wuya from Xiaolin Showdown, after falling victim to the Sorting Algorithm of Evil and becoming a minion of Chase Young. She's so reliably untrustworthy it makes her MORE predictable, and he incorporates her inevitable betrayal into his plans.
Jafar actually lampshades Iago being a reliable traitor as a compliment since it fits into Jafar's plan of revenge against Aladdin, though Jafar's ownership of the Villain Ball and Contractual Genre Blindness does him in when he forgets/ignores the possibility of Iago taking one last turn on the ol' Heel/Face wheel.
And at the end of the third movie, he takes a third option and flies off with Aladdin's father, the King of Thieves. "You're a good guy, Kazim. But not too good."
The Secret Saturdays had Doyle's girlfriend betray him for Van Rook in her second major appearance. What makes her this trope is that she betrays Van Rook next!
Winx Club's Trix started falling into this role after they becames returning minions. Darkar they actively betray, combining themselves into one super-being and helping the Winx because they think it will help their situation if they team up with their mortal enemies against the big bad it didn't, as they got abandoned in Realix once the battle was over. In the next season, they simply stay out of the final fight with Valtar, deciding to spend the time trying to figure out why they let other people boss them around.