Irrevocable Order

Some orders are meant to be carried out when given, no matter what. That's great for making sure the job gets done, and it prevents someone pretending to be in charge from cancelling it, but if the order actually does need to be cancelled... well, someone's going to have a bad day. If the order goes through and someone dies, you can expect someone to say "My God, What Have I Done?". In less serious cases, it's Played for Laughs as the characters scramble to stop things.

Self-destruct mechanisms, bombings, and assassinations tend to be the most commonly used implementations of this trope.

See also Irrevocable Message, Rhetorical Request Blunder, and Can't Stop the Signal.

Warning: Many of these examples are spoilers.

Anime and Manga

 * In Canaan the vice president cannot call off a B-2 about to bomb the president.
 * Code Geass: Every order given by Lelouch through his Geass must be followed if the subject is able, and, since it only works once per person, cannot be taken back. Really comes back to bite him
 * Played for Laughs in D Gray Man. Several of Komui's wacky series of robots, the Komurins, seem to lack a function to abort any given command. It only seems that the Komurins can prioritize certain commands, as seen when Komui has one go after Allen rather than have her sister subjugated to its augmentations. By the time Komurin-Z, a walking Mazinger parody, first appears, Komui finally seems to have gotten the point and installed a "terminate command" option.
 * In One Piece, Spandam is given the normally admiral-only privilege of initiating a Buster Call, where ten large battleships led by five vice-admirals arrive at a location and bombard the place with explosive cannonballs until the place has been leveled and no survivors remain. Spandam accidentally signals the Buster Call to the island where he lives and works, and no command exists to rescind it. From then on, the Straw Hat Pirates' main objective shifts from defeating Spandam to holding up against the Marines and getting off the island.

Film - Animation

 * In WALL-E, Directive A-113 orders Axiom to prevent humans from attempting to recolonize Earth, since it's too polluted to sustain life. It can't be countermanded because the only guy with the authority to do so is long dead. There is the non-sanctioned method of having the captain to get out of his chair, walk to the control panel, and press the "manual control" switch, quite a feat to say the least. It Makes Sense in Context.

Film - Live Action

 * In the first Alien film, Ripley sets the self-destruct for the Nostromo in order to destroy the alien when fairly certain that she can safely get to the escape shuttle. However, along the way, she finds that attempting to do so will put her directly into the path of the alien, so she tries to go back and shut down the self-destruct. But, by the time she goes through the ridiculously-complicated abort process, the point of no return has already been passed.
 * Bulworth has the title character lay a hit on himself (he's suicidal). For obvious reasons, he cannot call off the hit. He manages to call it off eventually, but only by unknowingly dating the one girl who was actually assigned to assassinate him, and accidentally telling her about the mistake.
 * The plot of Dark Star centers on an active bomb that refuses to drop from the bomb bay and cannot be shut off.
 * The black comedy Dr. Strangelove and the serious drama Fail Safe both revolved around this in the context of the Cold War. In Fail Safe, an instrument failure during a military exercise causes a squadron of US bombers to set off on a real mission to nuke Moscow; in Dr. Strangelove, it's an insanely paranoid general who deliberately gives all his bombers the orders to attack the USSR. In both films, due to the intense security procedures involved, the government is unable to call them back.
 * In The End, Burt Reynolds plays a man with a terminal disease who wants to kill himself; he gets another man (Dom DeLuise, a psychiatric patient, to help him. Near the end of the film he decides not to kill himself, but DeLuise's character continues to try to kill him. He thinks it's a No Matter How Much I Beg situation even though the man never told him that.
 * Intolerable Cruelty: the lawyers find they cannot reach the killer they hired to kill one's ex-wife (they're in a divorce, and the hit was dicovered was ordered when it turned out her former husband was poor, and therefore so was she. When another ex-husband goes Out with a Bang leaving an unchanged will that leaves everything to her, they try to cancel the hit). So they end up trying to stop the killer themselves before it's too late.
 * The Odd Job: which was originally a half-hour comedy episode starring Ronnie Barker and David Jason, later made into a film starring Graham Chapman. Same basic plot - guy decides to end it all by hiring a hit man to kill him when he doesn't expect it, but then finds he has something to live for after all and desperately tries to find the guy so he can cancel the contract. He succeeds, but then falls foul of one of the assassin's booby-traps.
 * In Parting Shots the main character has a hit called on himself (as he has cancer and plans to kill everyone who ever wronged him before having himself murdered to leave the Love Interest with his life assurence money). On discovering he's recovering he tries and fails to call off the hit. Luckily the hitman misses and kills a hated dictator (and also framing himself for the protagonist's murders). As a result the protagonist gets off scot-free and the hitman becomes a hero.
 * In Tomorrow Never Dies, The Royal Navy launches a Tomahawk missile to destroy a terrorist arms depot. They quickly find out that there are nukes at the camp. But the missile is already out of radio range, requiring James Bond to go in and remove the bombs.
 * The Star Chamber revolves around a group of judges who hold tribunals for criminals who have escaped justice in the courts and who delegate Vigilante Execution to the mob. A major plot point is the inability to call off a hit on a couple of lowlifes when they turn out to be innocent of child-killing.
 * Star Trek II the Wrath of Khan. After Khan activates the Genesis Device Kirk decides to beam the device aboard the Enterprise to disarm it. His son David, who helped build the device, tells him he can't.

Literature

 * Famous examples of this are from The Bible, in reference to the kings of the Medes and the Persians. The Medes and Persians had a law that if the king's ring was used to seal a proclamation then it could not be undone, not even if the king changed his mind.
 * Daniel and the Lion's Den is probably the most famous. King Darius made a decree that anyone who prayed to a God other than him for a period of a week would be fed to the lions—and sealed it with his ring. Daniel continued to pray, and despite Daniel being the King's favorite, and the King not wanting to go through with it, Daniel was still thrown to the lions.
 * Esther is another example. The Persian king gave Haman his ring, which Haman used to seal an order authorizing on a certain date the murder of all the Jews and the seizure of their property by the killers. When the king discovered Haman's plot, he had Haman executed, but could not undo the order. So he wrote out a new order allowing the Jews to kill anyone who attacked on that date. The Jews then slaughtered their enemies who attacked them.
 * In the Game of Thrones book, A Song of Ice and Fire, the dying King Robert tried to call off the assassination he ordered on Daenerys Targaryen, but some made sure his message didn't go through. While the assassination does fail, the attempt does a lot to motivate Dany to invade Westeros and retake the throne.
 * Joanne Nicole in The Tomorrow Testament used this as a Dead-Man Switch. The unmasked instigator of Humans-Drac war could blow up the station where peace negotiations were conducted, but was informed that a strike fleet was sent to bombard his homeworld and the only one officer in position to countermand the last given order in time is on board.
 * Happened in Elfsong by Elaine Cunningham -- Elaith politely informed the would-be victim that he changed his mind on the issue, but the executing agency isn't confirmed to be called off. And then thought a little more and chose to fight the hitmen he sent. He's funny like this sometimes.

Live-Action Television
"Davros: This is Davros. Elite unit seven will go to the incubator room. All survival maintenance systems are to be closed down. The Dalek creatures are to be destroyed. Doctor: Tell them the order cannot be countermanded. Davros: This order cannot-- Nyder knocks out the Doctor from behind Davros: This is Davros, this is Davros. My last order is cancelled, repeat, cancelled. No action is to be taken."
 * At the end of the third season of Castle, a major blow is dealt to whoever ordered Beckett's mother killed when his favorite hired gun is killed. The one who did that killing sent off a bunch of info to a fourth party so that there would be no retaliation against Beckett. Unfortunately, that mail arrived too late to prevent a sniper taking a shot at her.
 * Chuck: In Chuck vs. the Business Trip, an assassin known as "The Viper" is known for going dark after receiving instructions and making it impossible to retract an assassination order, meaning that Decker cannot call off the hit on Morgan.
 * Doctor Who: The Doctor almost gets Davros to do this in "Genesis of the Daleks." The Doctor has control of Davros' life support system and tells him to give the order to destroy the proto-Daleks or else he'll turn it off.


 * An episode of Seven Days had terrorists give a bomber wing an order to destroy an American base (supposedly overrun). Since Frank temporarily lost his memory when traveling back, the good guys didn't contact the bombers until they were past the official point of no return. They had to put the commander's pregnant wife on the phone to convince him.
 * Star Trek
 * Star Trek: The Original Series episode "Let This Be Your Last Battlefield". Rather than let Commissioner Bele take the Enterprise to the planet Charon, Captain Kirk orders the computer to activate the ship's Self-Destruct Mechanism. During the last thirty seconds of the countdown, Kirk tells Bele that once the countdown passes five seconds the computer cannot be stopped from destroying the ship. Bele gives in at the last second (literally) and Kirk aborts the self destruct sequence.
 * In an episode of Star Trek: Deep Space Nine the station's auto-destruct sequence (which was set up by the Cardassians before the start of the series) is set off. They call on Gul Dukat, the former station head, to disable it. He gives the turn off code, only to find out that his superiors had disabled his authorization to call it off since the only reason he'd have to call it off is if the station had been overrun by Bajoran resistance fighters and were forcing him to do so.
 * A Star Trek: Voyager episode features a sentient torpedo who's been sent to destroy the enemy. En route a peace is declared, but the command to the torpedo to abort the mission is garbled such that the confirmation code can't be immediately accessed. If the torpedo destroys its target, it'll set off another war. Our heroes must solve this problem. There's also another group of fully-active torpedoes that did receive the order, but had already reached the point of no return and disregarded it.
 * In a Kojak episode, a hitman refuses to cancel a hit on someone, and kills his client when he threatens to spread the news to the underworld that he's unreliable.
 * Sherlock, "The Reichenbach Fall":

Theater

 * Edmund from King Lear tried to call off the execution of Cordelia, but is too late. Also counts as a Deadly Change-of-Heart.

Video Games

 * The Fallout series:
 * Part of the intended ending for the original "Van Buren" Fallout was a nuclear missile launch that couldn't be stopped about to eliminate what remains of human life After the End and the PC would only be able to stop a number of missiles based on their repair skill, and must choose which places are struck (alternately, you could have blown them up in the silos, but take yourself with them).
 * The Fallout: New Vegas DLC "Lonesome Road" has an attempt at this by Ulysses. He prepares some nuclear missiles to launch. After the player defeats him, it turns out all you can do is change the target; you can't prevent the launch, so you must either launch the missiles at the NCR, the Legion, or both... unless you rescued ED-E, and order the bot to perform a Heroic Sacrifice.
 * Averted in Tribes: Vengeance, where Mercury's hit on Daniel is called off just before he takes the shot.
 * Inattentive players of 4X games can give these.
 * In Sword of the Stars, Human and Tarka ships can't be given new orders while they're traveling FTL. If a Human or Tarka player signs a Ceasefire with another race while one or more of their fleets are en route to the other race's colonies they will continue and start a battle.

Western Animation

 * On Adventure Time, Ice King hires a hitman to get Finn and Jake. Only he misunderstands the concept of hitman (he thinks he's just going to hit them), and when he realizes that the hitman is actually going to kill them, Ice King tries to stop him, but the hitman proves implacable.
 * In Avatar: The Last Airbender, Zuko hires "Combustion Man" to kill Aang. He later has a change of heart, but even when he offers the assassin additional money to call off the hit, the silent man shoves him away and continues to pursue Aang.
 * Parodied in the Camp Lazlo episode "Taking Care of Gretchen". Sampson offers to "take care of Gretchen". Lazlo accepts, thinking "take care of" means pamper. After watching a mob movie, he realises what Sampson meant and tries desperately to stop what he now thinks is a hit, only to find he is too late.
 * They parody this in Family Guy. Peter accidentally orders a hit on Lois, so he goes to the mobster who contacted the hit man to get the hit called off, but just before the mobster can do it, the mobster himself gets killed in a hit.