Rash Equilibrium

Two characters, who are usually active antagonists to each other, make some sort of bargain. Both of them independently decide to break it. Often this is a sign of Genre Savvy on the part of everyone concerned.

A common result of the Scarpia Ultimatum. Also compare Hostage for Macguffin, a more specific situation where the heroes wait for the villain to betray them before doing likewise.

One common result, especially in more comic situations, is for one character to guilt-trip the other over the betrayal, before revealing their own cheating.

It's All About Me characters will regard their own cheating as beneath mention, but rage about the treachery of the other character.

The name is a pun on the game-theoretic notion of "Nash equilibrium", which in certain settings leads to this kind of behavior. See these pages on The Other Wiki for more details.

Examples of Rash Equilibrium include:

Film

In Star Wars: A New Hope, Leia lies to Tarkin about the location of the rebel base, and then discovers he was going to blow up Alderaan anyway, even if she told it to him.
This is par for the course in the Pirates of the Caribbean movies, especially the third one.
In Pat Garrett and Billy the Kid, Billy and a deputized ex-crook get into a Ten Paces and Turn kind of duel. When they start taking steps, Billy immediately turns, draws his gun, and waits. When the deputy tries to turn and shoot a couple of steps before reaching ten, Billy's ready for him.

Literature

The Silmarillion, immediately after the death of Fëanor:

"Then Maedhros ... persuaded his brothers to feign to treat with Morgoth, and to meet his emissaries at the place appointed; but the Noldor had as little thought of faith as had he. Wherefore each embassy came with greater force than was agreed; but Morgoth sent the more, and there were Balrogs."

The novelization of the strategy game Warhammer Fantasy Battle: Mark of Chaos had this going on with its Chaos protagonist. It's made clear from the beginning that he and his superior loathe each other since the latter is a sorcerer and he is a Khornate Champion, meaning he abhors most magic. When they get to the resting place of the MacGuffin, the sorcerer decides he doesn't need his champion anymore and turns around to incinerate him, only to notice and narrowly evade the axe headed for his head. The following fight ends after the initially losing Champion receives a gift of his god making him near-immune against all magic.
This happens in the Discworld novel Mort when Death and Mort are getting a countdown to begin their fight. Both of them attack on two.
In the Percy Jackson series Zeus, Poseidon and Hades - the Big Trio of gods exhibiting sibling rivalry of divine proportions - make an oath not to have any more children with mortal women, since said children become too powerful and could be used against each other. As even gods cannot be expected to keep it in their pants for too long, Zeus and Poseidon both break the oath and father children. (Hades had fathered children before they made the oath, and he hides them in a Place Beyond Time to protect them from the other two.) When the children or 'demigods' are found, it escalates the rivalry from divine proportions to... something even more divine.

Live-Action TV

There is a game show called Golden Balls that uses this trope, sort of. The two remaining contestants have a set amount of money which they have won and they can either share or steal the money. If they both share they get equal amounts, if one shares and the other steals then the one that stole will get all the money, if they both steal then they both lose the money. Of course a lot of the time both people steal after having a five minute discussion with the other contestant about how they're not greedy and they want them both to have money.
- This is pretty much the exact premise of the Prisoner's Dilemma
- See also Friend or Foe.
In Lost season 3, Ben agrees to send Jack home in return for Jack surgically removing his spinal tumor. Once into the operating room, Jack uses his momentary advantage to facilitate Kate and Sawyer's escape, risking Ben's life. Later, Ben reveals he never had any intention of letting Jack leave the island.
In season 3 of The Wire, Stringer Bell and Avon Barksdale, who've been partners since the beginning of the series, end up simultaneously betraying each other. Stringer gives Avon up to the police; Avon arranges for Stringer to be killed. Both continue to swear that they're brothers, reminisce, and embrace, even after their mutual betrayal.
In the Star Trek: Deep Space Nine episode "The Wire", one of Garak's many conflicting stories about his past has him and his friend Elim trying to frame each other for the same crime. Since Garak is Elim, it's not entirely clear what this is actually supposed to mean.
The Hustle episode "Picture Perfect" ends this way. In the episode, Mickey's team promises a forger a cut of their scam if he forges the fake Mondrian they need to make it work. The forger is already under a fraud investigation and pretending to be unfit to stand trial; he also says that in order to do make the forgery convincing he needs them to steal him a real Mondrian to study. When they go to pay him off, he says that if they don't give him all the money he'll report the theft of the Mondrian, but it turns out that Mickey let the investigators who are after him know that he's faking his medical problems.
In the Red Dwarf episode "Justice", Lister is being chased by a simulant, and they agree to meet face to face, without weapons; both violate the agreement. Lister's lead pipe isn't very impressive beside the simulant's BFG. Good thing for him that they're meeting in a "justice field", where any crime you try to commit is inflicted on you instead...

Professional Wrestling

This is standard procedure when two heels are going up against a face in a triple threat match, for obvious reasons. The heels always double-team the face at the start of the match... until one of them goes for a pinfall, the other pulls him off, they argue and then start fighting. When this happens, the face usually wins.
- One of the best examples is the Triple Threat match between Edge, Randy Orton, and Shawn Michaels in 2007. Before the match Edge and Orton, who were heel tag team partners, pledged not to fight each other during the match, and actually avoided this for the first 3/4ths of the match. When they finally turned on each other, they both tried to calm the other down, turned away to lure the other into a false sense of security, and started whaling away on each other.

Theatre

In Measure for Measure, Angelo tries to get Isabella to sleep with him in exchange for the life of her brother. She convinces his old fiancée to go instead under a Paper-Thin Disguise. He doesn't notice the difference, but orders the execution of her brother anyway.
Tosca has a very similar set-up: Scarpia tells Tosca that, if she sleeps with him, he'll tell the firing squad executing her lover Mario to fire blanks. After he sends out the order, she stabs him instead; it turns out that the order was to execute Mario normally.
Love's Labour's Lost carries this to the point of absurdity: the Prince of Navarre and his three attendants all swear an oath to stay away from women for three years, and all independently break it. The scene where this is discovered has three of them, in sequence, trying to guilt-trip those who've already had their oathbreaking revealed, only to be themselves exposed.

Video Games

In the last Genoharadan quest in the first Knights of the Old Republic, you challenge Hulas to a duel. While he tells you to Come Alone, he brings a small army of assassins with him and makes fun of you for listening to him. If you bring your party members along he hangs a lampshade on this trope.

Web Comics

Angel Moxie, in these three strips.
Schlock Mercenary has the mercenaries attempting to lure Breya's troops on board so they can capture her, while Breya attempts to get her troops onto Tagon's ship so they can take it over. Plan B... err C ensues. Then Tagon's employer shows up. Backstabbings all around.

Breya: We worked well together! We used to be friends! Is this how you repay that?
Tagon: You know, you'd be wracking my poor heart with guilt if your marines hadn't just attempted to hijack my ship.

Western Animation

In the final few episodes of the first season of Transformers: Beast Wars, Optimus and Megatron initiate a truce. Of course, neither side believes the other will keep their word, so while they both stick to the letter of the agreement ("no weapons") they unashamedly violate its spirit; treating us to several Slapstick fights and several humorous spying montages.
Stewie and Bertram's first confrontation in Family Guy:

Stewie: You came unarmed?
Bertram: As we agreed.
Stewie: Admirable - but foolish! (pulls out a gun)
Bertram: (immediately pulls out his own gun)

Real Life

Truth in Television with online dating: Both sides always lie and feel betrayed when they learn "SexyModel346" lied too.
- Also for any instance of mutual adultery. Shows up both in real life and in literature. The Great Gatsby is an example of the latter.
- It has happened that a couple each had their on-line lovers, made arrangements to meet, and discovered that their on-line lovers were their own spouses. Unlike Escape (The Pina Colada Song), this has invariably led to hostile divorces.
John Birmingham's (mostly) non-fictional He Died With A Felafel In His Hand: He and his housemate fall in love with the same woman, draw up rules of engagement for pursuing her and immediately start to break them.
Hitler and Stalin in World War II.