Advance to the Rear: During the opening battle, Heath sends Owen to return due to camp due to feeling that his "horse magnetism" might cause the order of battle to fall apart but he passes Brackenbury, causing his horse to start following Owen, which causes the entire army to follow Brackenbury and retreat before the battle even starts.
At the end of the second Reiko the Zombie Shop movie adaptation, the title heroine confronts the bad guy who intends to resurrect the witch who was killed by Reiko in the first movie. The bad guy is at Reiko's mercy and she's armed with a dagger, so what do you think she does? Stab him? Wrong! She uses her trademark power to turn his victims (including a cop) into zombies who attacks the bad guy AND Reiko, then she grabs the zombie cop's gun and shot the bad guy to death, only to discover that by turning the victims into zombies, she also resurrected the witch who escaped while she was fighting. But wait, there's more: the last scene reveals that after Reiko left, the witch resurrected the bad guy and they are now working together. And guess what's worse? This cliffhanger is never resolved! The third and last movie adaptation deals about a different story, so we are left to assume that the two evildoers are still at large, thanks to Reiko being too dumb to use a dagger!
At the end of The Ring, we find out that not only has the creepy girl from the well not been defeated, she's actually been released to reign terror upon the rest of the world. Plus she never sleeps, so it's not like we'll be getting a daily eight hour break from the reign of terror.
In the American/Japanese science fiction/monster flick The Green Slime, the space station becomes infested with monsters made of the titular stuff because a visiting Jerkass/The Kirk had smashed on the ground an unauthorized sample of mold taken from an asteroid. He'd done this within a huddle of other astronauts, so naturally a fragment of the container (complete with a blop of slime) gets caught in one of the astronauts' pant-leg, and it naturally feeds on the radiation during the decontamination process.
After Nedry shuts down the fences to steal the embryos and make his escape, the Tyrannosaurus rex gets out and wreaks havoc. In order to undo what Nedry has done, Hammond wants the computers shut down and restarted. When Ray Arnold initially refuses, Hammond insists: "People are dying. Would you please shut down the system." Arnold does so. Nedry was smart enough to program the raptor fences to stay operational during his sabotage; the total system shutdown releases the Velociraptors. Prior to this, the T. Rex had only killed one person and likely preferred snacking on its fellow dinosaurs as opposed to stringy, bony humans; the raptors kill two humans for the simple sport of it, and relentlessly pursue the main characters for the rest of the film.
When Ellie Sattler turns the power back on it also turns on the electric fence that Dr. Grant and the kids are climbing down. Dr Grant and Lex successfully gets down but Tim gets electrocuted and flies to the ground. Luckily he survives.
When the T. rex breaks out and attacks the kids, Dr. Grant gets its attention with a flare and then throws it away, causing the T. rex to forget about the kids and chase after the flare. So far, so good, except Ian Malcolm tries to help with his own flare, which only causes the T. rex's attention to be turned back to the people instead of Grant's flare. Malcolm's own attempt to throw the flare away doesn't work because he's running while doing so, so the T. rex focuses on the larger moving target. The result is that it keeps chasing him, leading to Malcolm being injured and Gennaro killed.
Also the heroes of the sequel, who are responsible for many of the deaths in the movie.
And yet again in Jurassic World, in which all the carnage is the result of a series of unfortunate events beginning with the supposed hero accidentally letting the Indominus rex out of its enclosure because he wasn't willing to wait five minutes for his boss to check its tracker.
In Big Game, Herbert is brought to National Security Vault to advise on saving the president. The person who had this brilliant idea has just unwittingly put The Mole in place where he can do the most damage.
In Hellboy (2019), when Hellboy, in his desire to avenge his father's death, pulls Excalibur from its stone and thus turns into the Destroyer Of All Things, his transformation opens the gates of Hell and unleashes scores of murderous demons on London's civilian populace. It lasts only a minute or two at most, but that's enough to result in countless extremely violent deaths among innocent bystanders. It's downplayed, however, because Excalibur was Hellboy's only option to defeat Nimue for real.
In Speed, while under the bus in a failed attempt to disarm the bomb, Jack has to punch a hole in the bus's gas tank with his screwdriver as a makeshift grip to keep himself off the ground to avoid being run over when his cart breaks. By causing the leak, Jack has now significantly cut down the time he and the LAPD have in trying to find a solution to end the crisis. When he informs driver Annie about it once safely in the bus, she replies in a panic, "What, did you feel like you needed another challenge?!"
Also, when the injured driver is being evacuated, the police on the flatbed try to get an old woman on the bus to come too, despite warnings from the bomber that he won't allow it. She tries to leave, but a bomb is triggered beneath her feet, dropping her under the wheels of the bus.
During the climax of Dogma, the fallen angel Bartleby needs to become human so he can take advantage of plenary indulgence in order to return to Heaven. Thus proving God wrong and thereby unmaking all of creation. To do so, he needs to remove his wings... which is promptly done for him with a machine gun by a particularly clueless hero who was trying to kill him.
Every protagonist from the Saw films (with the exception of Saw I and Saw VI) suffers from this, the worst case being Matt Gibson from Saw 3D.
In Immortals, Theseus acquires the Epirus Bow, but shortly after getting it, he drops it. The weapon is then taken by a dog (don't ask) and delivered to the Big Bad, who then uses it to great advantage. Had it not been for the gods' Big Damn Heroes moment at the end, his evil plans would've succeeded entirely.
In The Matrix Reloaded, Neo encounters the Architect on his journey to destroy The Matrix. He learns that, if he proceeds, every man, woman, and child connected to the Matrix would die, which, combined with the destruction of the rebellion, would be The End of the World as We Know It. Neo does it anyway. It got better, though.
By the way, Neo destroyed Agent Smith in the first movie; however, the latter came back as a rebellious program.
We find from the final film, this was really a Batman Gambit by The Oracle, so Neo was filling his role exactly as intended.
Also, in Harry Potter and the Philosopher's Stone since Dumbledore had enchanted The Mirror of Erised to only give the stone to someone who didn't want to use it Voldemort couldn't have retrieved it until Harry showed up and the stone appeared in his pocket. Furthermore, since Voldemort couldn't get the stone anyway, Harry and his friends' quest to stop Voldemort was unnecessary and they put their lives in danger for nothing. Actually, all they did was kill Quirrel.
Not just that, they set off an order of events that would eventually lead to Harry's own death (and every other death in the series). Voldemort was stuck in Quirrel, living a half life, he wasn't exactly thriving. By beating him in that form at that time he didn't allow Dumbledore time to find his horcruxes (which would have prevented him from respawning) and opened the door for him to come back as a complete (more or less) being and lose his inability to touch Harry. And it's not like Voldemort could've got the stone since even if he got wise and tried to use someone else to get it with the Imperius curse, they still wouldn't be able to get it because the second he tells them to get it they will want to get it and will therefore not be able to get it. He was stuck in that form and Harry did him a huge solid.
In Frequency, John manages to use the time-travel radio to keep his father, Frank, from dying in the burning building that had claimed his life thirty years ago. And so doing began a chain of events that caused a Serial Killer to live instead of die who then goes on to murder many more women including John's mother. Oh, and did I mention Frank still dies from lung cancer because he smokes?
The rest of the movie is spent trying to stop the serial killer from killing his other victims, including Frank's wife. In the end, the killer, who is still at large in the present, breaks into John's house and attacks him while John's talking to his father on the radio. Right as it looks like he's going to win, in walks a thirty-year-older Frank with a shotgun. He'd heard the fight on the radio, and thus knew exactly where the killer was going to be thirty years from now.
In Batman Begins, Bruce Wayne's desire for justice for Gotham leads him on a personal quest where he's declared dead, leaving Wayne Industries in the business of (among other things) weapons development. One device of Wayne Tech is stolen and used against the city. If Bruce had managed his company, that device might have never been made.
The Dark Knight: Batman's destruction of the Falcone family's dominance of Gotham City leads to not only a myriad of gangs trying to fill the gap, but also the rise of The Joker, who creates far more chaos and destruction. There are obvious real life parallels to international politics to when a power vacuum is created (collapse of the Soviet Union leading to rise of nationalism and fundamentalism) and law enforcement (the weakening of the Mafia that clears the way for Chinese, Russian, or Columbian gangs, for instance).
After Reese has run some numbers, Lucius Fox makes him do it all over again to teach him a lesson. This second, closer look leads to Reese discovering the identity of the Batman.
The Dark Knight Rises: Bruce Wayne dilly-dallies with nuclear fusion technology as an energy source, not realizing he was manipulated to do this to create a fusion bomb by remnants of the League of Shadows.
Bruce choosing Miranda to be in charge of the fusion reactor project, unaware that she is actually Talia Al Ghul, which grants her the possibility of flooding the basement to completely give the heroes no chance of ever stopping the bomb in the first place.
The whole plot of The Dark Knight Rises is because Bruce "let Ducard/Ra's die" in Batman Begins. Talia, who used to have a rather strained relationship with her father prior to that, then seeks to accomplish her father's visions.
Planet of the Apes: The original had an astronaut landing in the future from a freak accident. The remake has the hero causing the freak accident that launches his ship even further back in time and giving his mutant lab apes free rein over a primitive world. Changes things considerably.
Rise of the Planet of the Apes could have been called "Nice Job Breaking It Hero: The Movie". In his attempt to create an Alzheimer's cure, Will sets into motion the eventual dominance of the apes, as well as inadvertently creating the plague that wipes out most of humanity.
John Woo's Broken Arrow sees the hero finding a nuclear warhead which the villain has left in a mine. It hasn't been armed yet, so he comes up with the idea to enter the wrong arming code three times, causing a security measure to lock the warhead so it can't be armed. He does so...only for the villain to mockingly inform him that he used uncoded circuit boards, "You have just armed a nuclear warhead, my friend." Oops.
Dorothy: My ruby slippers— The Nome King: No, no, no... My ruby slippers. They just fell out of the sky one day — you were so anxious to get home! They're very powerful: they made it possible for me to conquer the Emerald City... thank you.
Near the end of The Haunting in Connecticut, the reverend manages to exorcise the ghost from the house. Too bad he's a benevolent spirit who was preventing the dozens of malicious ghosts from wreaking havoc.
In The Monitors, after the heroes drive off the dictatorial machines that enslaved mankind and stopped all human conflict, wars start breaking out all over the place.
In Skyfall, M gave Silva to the Chinese to save 6 agents, but in the process caused Silva's Start of Darkness, leading him to kill dozens of MI6 staff later on. Also, Q's attempt to decrypt Silva's computer by plugging it into MI6's network ends up releasing a virus that opens every door in MI6's new base, allowing Silva to escape.
At the end of Annihilation: Earth, David is torn between his idea of shutting down the particle colliders to stop the apocalypse or turning them Up to Eleven, as his Middle Eastern colleague Raja suggests, which will (supposedly) force the anomaly to be "snuffed out." His boss makes him doubt Raja's motives, and David ends up going with his original plan. Cue the literal Earth-Shattering Kaboom. The moral of the story: not all Arabs are terrorists.
The Hunger Games: Mockingjay Part 1 has Katniss tearfully telling the cameras that the Capitol was responsible for the destruction of a hospital full of innocents. While the Capitol planes were showing aggression, it was actually Katniss and Gale who shot down the planes, causing them to smash into the hospital and completely destroy it.
In Dragonheart, the knight Bowen is reluctant to kill Draco, even though it's the only way to save the kingdom from its Ax-Crazy king Einon, because Draco is the last dragon left in the world. Whose fault is that, might you ask? Why, Bowen's, who spent ten years on a personal crusade to wipe out the species!
Eh, that was part of it, the other part being that Draco had become his best friend. See how eager you are to kill YOUR best friend when the chips are down. No, the real hero who did the breaking is Draco himself, by giving half his heart to a monomaniacal Jerkass he created an immortal dictator that made life a misery for his subjects.
The Chinese in the sequel. First, they slaughter all their dragons because one of them turned out to be bad (whom the others immediately stopped and punished). Then they take the heart of the bad dragon to England, which results in the Big Bad (the same dragon) restoring his original form.
This trope kicks off Dracula Has Risen From His Grave, in which a couple of priests go to Dracula's castle to exorcise any lingering evil left over from the previous Christopher Lee Dracula flick. One of the pair slips and falls on some rocks, and blood from his resulting scalp wound drips down to where Dracula's inert body is lying, reviving the vampire for another round of mayhem.
Heroic Trio sees an evil sorcerer kidnapping babies to raise as an evil army. One of the main characters kidnaps a baby in order to lure the real kidnapper out. This results in a fight between a fellow hero who mistakes her for a villain and to make matters worse, the real kidnapper shows up to fight both of them. The baby ends up dying in the process.
Tremors 2: Aftershocks: While trying to escape shriekers, the 1st evolution stage of a graboid, they come across one in the way to an escape truck. So Burt Gummer, resident gun nut, whips out a BFG and blows it to bits. The problem? He was prepared to shoot at an underground creature, not the small above-ground shrieker. The bullet from the gun goes through the shrieker, a wall, several barrels of oil, and finally, the truck's engine. Burt's response? "How could I have known?!" note He couldn't have known. The shrieker was in front of a concrete wall.
In The Mummy: Tomb of the Dragon Emperor, Lin, who had been the guardian of the tomb and tried to prevent the emperor from escaping, was responsible for knocking the diamond into the emperor that awakes him as she attempts to attack his decoy.
In 28 Days Later, an Animal Wrongs Group releases the Rage Virus when trying to free some filthy monkeys. This is after they are directly told by a scientist that the monkey "is infected with 'rage' and all it takes is one bite".
The rampant idiocy of the cast of 28 Weeks Later makes those guys look intelligent. First the Swiss Cheese Security of the compound allows people to leave the safe zone at will, leading to the discovery of a virus carrier that, for some reason, they bring into the safe zone and leave in an unlocked room with no guards to speak of. When someone inevitably gets infected, their "logical" decision is to herd the population into a poorly guarded room and shut all the lights off. Naturally, it gets worse when it leads to nearly everyone dead or infected. Then at the end, the boy becomes a carrier and his sister neglects to ''tell'' anyone, which results in the infection decimating France and possibly all of mainland Europe.
In Blade, the hero doesn't kill Quin completely, allowing him to revive himself in a hospital to turn Karen and kill her boyfriend. Since Blade shows up immediately after this happens, looking for Quinn, he apparently knew that Quinn wasn't dead. His reasons for leaving Quinn alive in the first place are never explained, leaving a plot hole.
Transformers: Dark of the Moon: If Optimus Prime hadn't discovered the Ark on the moon, he wouldn't have been able to use the Matrix to revive Sentinel, allowing the traitor to kill Ironhide, steal back the Space Bridge Pillars, and join Megatron and work on enslaving humanity as a labor force to rebuild Cybertron.
Charlotte Mearing: (After Sentinel ruins the NEST Base.) "Yeah, take a good look, Optimus! This is all on you!"
In Legend (1985), one of the "heroes", Lily, breaks it by touching a unicorn, even after Jack tells her not to, resulting in the ice death of the world. It gets better. Eventually.
In Insidious, the twist ending is revealed when Elise takes Josh's photo, dooming herself and most likely Renai as well. On second thought, Josh is guilty of this as well.
In Memphis Belle, the co-pilot begs for a chance to take some shots at the attacking German fighters, and is elated when he downs one of them ... only to watch in horror as it crashes straight into the tail of the rookie crew's bomber.
In Men in Black II, J neuralyses Newton at the video store, and gives him an instruction to not live with his mother, as he was a middle-aged man. It is heavily implied (by him calling for his mother while brandishing a shovel) that he kills her to follow this instruction from J.
In I Am Legend, Dr. Alice Kripin creates a mutated strain of the Measles virus which cures cancer ... and then promptly mutates again into a much worse virus that wipes out 90% of the human race and causes 99% of the survivors to be mutated into excessively violent vampire/zombie creatures which then slaughter all but a handful of the remaining uninfected people.
The 2012 The Three Stooges film is about the trio trying to raise $830,000 their orphanage is in debt for. It's later revealed that they're in debt because of insurance bills caused by injuries from their antics over the years.
In Mystery Science Theater 3000: The Movie, Mike is goaded by Crow and Tom to pilot the Satellite of Love, despite being warned by Gypsy not to do so. Mike brushes her off, starts it off smoothly... then promptly runs into the Hubble Telescope. Then he tries to pull it off with the manipulator arms, tearing a part of it off wiping sweat off his forehead. When he finally releases, it suddenly drops to Earth as a fireball! Mike lampshades the improbability of the last one.
Ming claims to only destroy planets that are advanced enough to realize his attacks aren't natural disasters...hence, it's Zarkov's own attempt to contact whoever was launching the attacks that nearly dooms the Earth.
Zarkov also gets Flash killed. Good going. Flash gets better though.
First film: Saves his dad from getting hit by a car—- which as a result keeps his parents from meeting, and in turn, is causing himself and his siblings to slowly be erased from existence.
Second film: Purchases a sports almanac—- which Biff steals and delivers to his past self in 1955, allowing a new power hungry Biff to turn the post-1955 era into a Bad Future. And in another case of breaking things, Biff managed to use the Delorean to go back to the past just because Marty happened to be looking away from it for a few minutes. Also an example for Doc Brown, who forced Marty to throw away the Almanac allowing Biff to find it.
Third film: Saves Doc Brown from getting killed by "Mad Dog" Tannen—- and now changes history so that he'll be the one getting killed instead.
At the end of Excision, Pauline tries to perform a lung transplant for her Ill Girl sister Grace. It goes about as well as you can expect for a complicated procedure performed by someone with no medical training, little proper equipment, and a garage for an operating room.
In Star Trek Into Darkness, Kirk stuns a creature that appears before him as he runs from a planet's natives, only for Bones to tell him that the creature was their ride out.
Furthermore, Kirk's lust for revenge is what ultimately dooms the Enterprise. If he had not desired vengeance against Harrison, there would have been no eventual confrontation with the Vengeance. He wanted vengeance and he got it. The Vengeance's merciless assault is what inspires his heartfelt apology to his crew, as seen in the trailer.
Khan's decision to crash the Vengeance into San Francisco and kill countless innocents was probably influenced at least partly by the fact that Spock had tricked him into believing that all of his crew were killed.
Admiral Marcus points out briefly that landing in Klingon territory and taking out several patrols (even in self defense) will still draw the wrath of the Klingons against the Federation. Even though this was his goal to begin with. It's implied this is the reason why the two are hostile to each other during the 5-year expedition of the Enterprise, which starts at the end of the film.
As a furious Scotty points out, by confiscating his transwarp equation, Starfleet inadvertently ended up allowing a rogue Starfleet officer to commit a terrorist attack and then jump across half the universe to safety.
Batman v Superman: Dawn of Justice: Superman overpowers Doomsday and drags him into space, where they can fight without hurting anyone. The President decides to launch a nuke at them, ignoring General Swanwick's protests that Superman has the situation well in hand. The end result, Superman gets knocked out and nearly killed, while the explosion knocks Doomsday back to Earth and makes him more powerful.
In Pacific Rim, Newton drifts with a partial kaiju brain, and learns that the kaiju operate on a hive mind and are being engineered. Unfortunately, he forgot that the drift works both ways, meaning the kaiju hivemind now know what he knows.
Also Stacker Pentecost grounding Gypsy Danger is what probably cost the lives of the two jaeger crews that died in the first double kaiju attack. If he had let the go out right away it's likely that all four of the last jaegers would have survived.
In The Lightning Thief, Percy slices off the Hydra's heads, not knowing that it would come back with ten instead of five.
The train carrying the nuclear warheads meant to take out the MUTOs is ambushed by a MUTO, who promptly eats one of them.
The Navy accidentally provoke Godzilla into destroy the Golden Gate Bridge.
Godzilla stops the flying Muto with a quick tail swipe into a tall building...which then collapses on top of him.
In Shin Godzilla, the US Armed Forces come to the JSDF's aid—and screw things up really badly. How bad? Godzilla's reaction to bunker busters wounding him is to unleash his Atomic Breath for the first time, irradiating half of Tokyo and killing the Prime Minister and most of the Cabinet when they try to leave the city For that matter, Rondo gets one for talking the Prime Minister into evacuating during said attack, putting him in the path of the Atomic Breath blast.
In The Return of Godzilla the Soviets aren't exactly portrayed as the "good guys" in either the Japanese or US cuts, but the nuke they launch on Tokyo (accidentally in the Japanese cut, and on purpose in the US version) would've wiped out Tokyo had the US not intercepted it in orbit. Even then, fallout from the missile's detonation is enough to rejuvenate Godzilla and allow him to kill the Super-X.
In the original Godzilla (1954), Dr. Serizawa expressed the fear that revealing the Oxygen Destroyer's existence would result in a threat worse than nuclear weapons before he uses it on the original Godzilla. Godzilla vs. Destoroyah sees that the good doctor himself unintentionally realized his own fears as the eponymous Destoroyah was created by the weapon's use against the first Godzilla.
The Distinguished Gentleman: The protagonist is a Con Man who's just scammed his way into Congress. The first time he votes, he doesn't even bother to find out what it's for, since he's having too much fun just being there. On a whim, he pushes "No". Then he encounters a public tour whose guide asks him about his vote and confronts him with the knowledge that it was a welfare bill. Oops.
Resident Evil: Alice and the assault team manages to shut down the Red Queen, only to realize that the Red Queen was attempting to contain the T-Virus infection within the Hive, and that they have now released hundreds of zombified animals and people into the facility.
Gwen stops Aberline from shooting Lawrence, which in turn causes him to get bitten. However, this really doesn't do much good since Gwen ends up shooting Lawrence herself in the end. Although, she's somewhat justified since she was still convinced she could reach Lawrence, and well... she was right. Plus, she probably figured that Lawrence would have finished off Aberline instead of chasing after her instead.
Whilst in the Hindu Kush, the locals who told Sir John about the feral child in the cave, thus causing him to get infected.
X2: X-Men United: Rogue's impatience makes her indirectly responsible for Jean's death, due to crashing the X-Jet and causing the vertical takeoff to go temporarily offline. Furthermore, since the jet appears to have been only about a few hundred meters from the facility, she really didn't need to fly to pick them up anyway.
Charles trains Erik, helping him to improve his power. This backfires when the latter decides to turn into Magneto.
Had Charles and Hank shown acceptance for Raven's true form, she might not have teamed up with Magneto.
Moira desperately shoots at Erik, forcing him to deflect the bullets, one of which paralyzes Charles right next to him.
Charles erases Moira's memory, clearly discrediting her within the CIA and possibly ruining her career. The fact that one of the few snatches of memory she has left is of their kiss is just the icing on the cake.
The heroes' decision to break Magneto out ultimately does more harm than good. Given Mystique's reaction to Charles's presence, it seems leaving Erik in prison would have saved a lot of problems. It's even lampshaded by Xavier when he tells Logan, "It was a mistake freeing Erik."
Magneto tries to kill Mystique because the medical experiments performed on her were used to give the future Sentinels their adaptive abilities. However, this attempt spills her blood, allowing it to be collected and sent to Trask Industries. Subverted in that Trask still needs the living specimen.
Depending on how you interpret The Stinger, it seems that in preventing the Bad Future, the X-Men have somehow inadvertently set the stage for Apocalypse to rise up and attack mankind in the new timeline.
The cultists had managed to locate En Sabah Nur's resting place, but they have made no progress in reviving him... until Moira left the entrance to the cave uncovered, allowing the sunlight in to power the machinery and waking Apocalypse.
Xavier's conversation with Magneto via Cerebro makes Apocalypse aware of the telepath's existence and he taps into the latter's powers.
Havok's attempt at saving the Professor from Apocalypse only manages to blow up a generator under the school, causing a huge explosion. Thanks to Quicksilver, the only casualty ends up being Havok himself, being the closest to the blast.
It's made clear during the climax that the AI was Will all along and all he wanted to do was fulfill his wife's dream of using technology to heal the planet. Had the Government not interfered, as well as Evelyn not give into her doubts, Will would have created paradise on earth. "Humans fear what they don't understand." Instead much of humanity is sent back to the Stone Age and countless millions likely died in process.
Max actually sides with the people that murdered Will to stop the machine impersonating him. They only succeed because RIFT threaten to kill Max unless Will destroys himself, a plan that only works because it's genuinely Will. Essentially Max enables RIFT to kill both his best friends and end a chance at a better world.
In Street Fighter, we have Carlos Blanka who's been captured by M. Bison and what does Colonel Guile do? Tell good old Charlie to "hang on buddy" right in front of Bison! Way to tell the deranged lunatic who has a personal investment in you that he has your friend right there and at his mercy. GuesswhatCharlie'sfateis.
In St. Vincent (2014), aside from hanging out with a prostitute, Vincent took Oliver to a racetrack and a bar while babysitting him. This ends up costing Olivers mother half-custody when Olivers father takes her to court.
In Pixels, one of the Arcaders uses cheat codes to win against Pac-Man, but all it succeeds in is enraging the aliens and speeding up the destruction of Earth.
In Follow the Fleet, Bake goes to a theatrical producer to try to get Sherry an audition for a show. He hears that the producer is already auditioning someone he really likes for the part, so Bake sabotages the unknown performer. What he doesn't realize is that Sherry has gotten an audition on her own, so she's the one he's sabotaging.
They never once mention to Kaulder that the Witch Queen's heart is still alive, which could have kept him aware of possibility that it may be used to bring her back one day, which is exactly what happens.
While the above is bad enough, the fact that they withhold the little fact of the heart being stolen, thus stunting Kaulder's investigation until it's too late, smacks of idiocy.
At some point, they switched from killing dangerous witches on spot to arresting them. While humanitarian and commendable, this creates world's most powerful coven for the Witch Queen to use for her spell.
The movie Toy Soldiers has a scene where Joey, one of the students, killed when he tries to inspire a revolution against the Colombians holding the students hostage. This causes the villain's plot to implode, thereby placing the student body in danger because Joey's father, The Don, orders the murder of Cali's father during a prison riot. Since Cali is holding the students hostage in exchange for his father, he orders the murder of the students as soon as he finds out.
In Ghostbusters (1984), at the climax, Gozer demands that the Ghostbusters pick the form of The Destroyer, Gozer's instrument to kill all life on Earth. Peter, Egon, and Winston all manage to keep from thinking of anything, but poor Ray had an idea - the most innocent thing he could think of from his childhood - the Stay-Puft Marshmallow Man. Cue 100-foot tall marshmallow golem rampaging through New York.
Rocky II: Rocky as the movie progress faces serious financial issues and has to accept the rematch with Apollo despite the concern he could potentially go blind if he suffers serious damage. All of this could have easily been avoided had Rocky not been so irresponsible with his money and blew it all away on things like motorcycles and jackets. Adrian at one point even tried to talk Rocky out of buying expensive jewelry for her so they could save money, but he would not listen nor did he bother to listen to Gonzo's advice about investing the money he made.
Deep Cover: After undercover cop Russell Stevens sets up a deal with drug kingpin Hector Guzman to record the transaction, he is followed by a local narcotics detective who scares off Guzman and almost sabotages the whole thing.
In the 2007 film adaption of Hairspray, when Tracy participates in the march and hits a cop on the head with her sign and runs away, it gives Velma a good opportunity to stop Tracy from participating in the pageant by having the same police guard the entrance, making Tracy and the others devise a plan to sneak into the pageant. If Tracy had been bailed out along with Maybelle, she wouldn't have had to hide from the cops and sneak into the pageant!
Blue Monkey: The entire thing is caused thanks to some stupid kids feeding a parasitic insect a bottle of powdered growth hormone. Up until then, there was only one casualty (the man initially infected with the thing), and the bug was safely contained in a jar. But after eating the hormone the kids give it, the thing grows bigger than a normal human and ends up siring more of its kind. The kids are consequently indirectly responsible the deaths of the nurse who'd been watching the specimen, her boyfriend and several other patients and staff.
Admittedly, the nurse herself is just as much to blame; the kids wouldn't have been able to have access to the specimen if she hadn't snuck off with her boyfriend to have sex right before they came in.
Snowpiercer: The protagonist himself does this twice. First by going on with the revolt which turns out to have been engineered by the train conductor as a way to cull the population and keep the train balanced, and going too far with that ending in more deaths than expected/needed, then again when he tries to intervene and help lil Timmy, which costs the other boy to sacrifice himself to keep the engine going. Then more egregiously the good guys epically mess up by blowing up the side door to the train, which causes an avalanche to cave in and derail the majority of the train, specifically the rear end, where the protagonist and his people have been suffering and trying to leave since the start of the movie.. at the end of the movie the protagonist is dead and while it's not confirmed whether all the other passengers died (except for the girl and the kid) the hero's people are definitely goners, and the train is in bad shape, and the majority of the people are potentially dead, and the only guy who knew how to survive outside is definitely dead, and there's a good chance the only living beings on the planet are a teenage girl, a little boy and a polar bear. You can guess how well that's gonna go for the humans.
Also done before the movie begins by the scientists who fired into the sky a special missile with a gas meant to reverse global warming. It worked...and caused a new ice age.
The US Air Force only made things worse in Shin Godzilla as it's implied Godzilla developed his atomic breath as a result of bunker busters injuring him, where he not only destroyed the bombers, but also lays waste to much of Tokyo and kills the Prime Minister and much of the Cabinet when their helicopter gets caught in its blast.
Yoda may have been a great military commander, but he sucked as an emotional counselor. In Real Life, it has been known since time immemorial that platitudes, no matter how well-meant, fall flat on the hurting. Unfortunately, platitudes were all Yoda had for a worried Anakin Skywalker. Darth Sidious promised actual help...
Even before that, Yoda decided to give Anakin a padawan because he believed that training a Jedi until she was fully able to take care of herself would help Anakin learn to let go of his loved ones, freeing him from the fear of loss that could lead him to the Dark Side. Unfortunately, Yoda couldn't anticipate that Ahsoka would choose to leave the Order before her training was complete, which only increased Anakin's fear of loss and inability to let go, pushing him closer to the Dark Side.
And even before that it's hinted that the reason behind General Grievous' Start of Darkness (and outright stated in Star Wars Legends) that the Jedi wronged him in some way, and with the Huk War still being canon, it's possible that the Jedi's accidental enabling of his people's genocide is as well.
The Last Jedi has one of the biggest examples in all of Star Wars.
Convinced that Holdo is just ready to take the fleet and fly off, Poe agrees to secretly let Finn and Rose go on a mission to find a splicer, get on the First Order main ship and deactivate the sensors letting them track the Rebel fleet through hyperspace. When Holdo seems ready to go, Poe leads a mutiny to stop her. At which point, Leia herself knocks Poe out and when he comes too, she tells him that Holdo was following her plan: send transports with the 500 Rebels cloaked from sensors and settle on a hidden base on a nearby planet. The cruiser will go on to lead the First Order away, unaware they're chasing a ghost ship. Holdo wasn't a coward, she was doing exactly what Leia would have.
Which really ends up blowing up in everyone's faces as Rose and Finn are caught before they can destroy the device. Then that "charming rogue" splicer not only sells them out in a heartbeat but also gives the First Order codes to track the transports. They soon destroy nearly the entire fleeing fleet and are only stopped by Holdo ramming the cruiser into their ship in hyperspace. But between that attack and a follow-up battle at the now-discovered base, a strong force of 500 people is down to barely thirty.
In other words, if Poe had just trusted Holdo and not had Finn and Rose do this (or, conversely, Holdo had kept Poe in line), the plan would have worked and the full Resistance would have been safe. If that's not living up the trope, few things do.
Holdo herself. If Holdo had discussed the plan with her senior officers, rather than keeping it a secret, not only would she have forestallled the mutiny, she would also have benefited from their input and ensured that, even if something happened to her, other people would be able to execute her plan. By keeping it a secret for no apparent reason, she set a chain of events in motion that quickly spiraled out of control.
Earlier on when Luke sense a growing darkness within Kylo Ren, the former actually contemplated on killing him in sleep but even though he realized his mistake, the damage has already been done and it ultimately led Kylo Ren to believed that Luke actually intends to murder him and it ultimately ended up leading him to defect to the Dark Side and kickstarts the plot of the sequel trilogy.
The Cloverfield Paradox reveals that the the first successful firing of the Shepard supercollider to in an attempt to solve the Cloverfield Station crew's world's energy crisis not only wrecked their own world, but by implication of the titular theory, The Multiverse, including the worlds of the original Cloverfield and 10 Cloverfield Lane.
Mythica: Because Marek is a necromancer, every time she touches a part of the Darkspore, it alerts Szorlok to that part's location.
In The Blues Brothers 2000, Elwood's well-intentioned attempt to deal with the Russian mob promptly gets Willie's club burned down.
In The Guilty, Asger's attempts to help Iben, a kidnapped woman, end up making things worse:
He tells Iben to put on her seatbelt and pull the handbrake to incapacitate Michael, her ex-husband and kidnapper, so she can get away from him. It doesn't work, and instead results in Michael putting her in the back of the van.
He tells Iben to grab a brick and hit Michael over the head when she gets the chance. She does, but since Michael never intended to hurt her, only bring her to the psychiatric ward where she could get the help she needed, it doesn't make anything better. Instead, Michael gets a concussion before he can bring her to the psychiatric ward and Iben herself goes missing.
In The Gravedancers, the protagonists dig up the physical remains of the ghosts—almost getting killed in the process—so they can rebury the bones and lay the ghosts to rest. However, after reburying the bones, the ghosts return, even angrier than before. It is then that Frances reveals that she stole the skulls before the bones were buried so that the ghosts would hang around and she could properly document the existence of the supernatural.
In Sonic the Hedgehog (2020), the movie's central conflict happens because Tom shoots Sonic with a tranquilizer dart, accidentally sending all of Sonic's rings to San Francisco. The resulting road trip from Montana to California is because Tom agrees to help Sonic retrieve the rings. On top of this, Tom also handed over one of Sonic's quills to Robotnik since Tom just left it lying around his house instead of hiding it in his pockets.
Shrooms: While Jake's reasons for confiscating the group's mobile phones is understandable given the nature of the trips they could have after ingesting the mushrooms, it still isn't exactly a wise idea (arguably). To be fair to him, though, he wasn't to know that the phones would get stolen...
In Ip Man 4, as well-intended as he was, Hartman's attempts to bring Chinese kung-fu to the military by going over his superior Geddes is what spurs Geddes to send Frater to cause trouble in Chinatown, only for Geddes himself to later attack the Chinese Benevolence Association when it wasn't enough.
Deadtime Stories: In "Peter and the Witches", Peter kills Hanagohl to prevent her from sacrificing Miranda and resurrecting her sister Magoga. However, Hanagohl falls on top of Magoga and her blood dribbles into her sister's mouth: completing the ritual and resurrecting Magoga.
In God Told Me To, Pete publishes a newspaper article about the murderers' religious motivation, hoping that once they're acknowledged the killings will stop. He only succeeds in causing panic and riots.
The Hobbit: When he encounters Gollum in the cave, Bilbo says he's Bilbo Baggins of the Shire. 60 years later, this information will be tortured out of Gollum by Sauron's soldiers, alerting Mordor to the One Ring's location and forcing Frodo to flee the Shire.