Bee Movie: Layton: "This is an unholy perversion of the balance of nature, Benson! You'll regret this." Needless to say, the defense attorney was absolutely right, as Benson's victory did upset the balance of nature. Fortunately, Benson did fix it before it became irreversible.
The Lorax: The Once-ler starts off with good intentions when he makes the Thneeds, like not polluting the water, not clear-cutting everything, not consuming all the food, and not terraforming the forest into a wasteland. Now......
Made even more significant when he didn't have to chop all the trees down in the first place. All the harvesting was at the top meaning the wood wasn't needed at all, unless it was the material to build the industry.
Mulan's grandmother gives her a cricket to give her luck in order to help her with the matchmaker. While Mulan wasn't performing well to start with, it's obvious that the cricket ends up being a main direct cause for her failing the matchmaking.
Compare the fate that would have awaited her if she had succeeded in her matchmaking with what happened after she failed. Had she succeeded, she almost certainly wouldn't have joined the army in her father's place, making it very likely that not only would he have died, the Huns would have managed to succeed in their invasion. She also would not have earned the respect of the Emperor or met and won the heart of Shang, who is a better match than the matchmaker could have arranged for her. Who said that cricket wasn't a lucky one?
In 9, the title character's newfound friends take out the mechanical beast who had been threatening them, only for 9 to realize that the MacGuffin he'd woken up with fit perfectly inside a larger machine nearby. Said machine proceeds to kill 2 immediately, followed by 8, 5, 6, and 1 before it's all done.
Not only that, but what he did was exactly what said mechanical beastie was trying to do not five minutes before. Too Dumb to Live.
Also, the Stitchpunks blowing up the factory. Beforehand the Fabrication Machine had been Sealed Evil in a Can, being confined to said factory. However, instead of being destroyed in the explosion, the Fabrication Machine was essentially freed, and proceeded chase after Stitchpunks itself, leading to the deaths of 5, 6, and 1.
Ralph's own game-hopping almost gets Sugar Rush devoured by a horde ofCy-bugs, and turns Big BadTurbo into a One-Winged Angel capable of taking down the whole arcade. The latter even calls him out, gloatingly thanking him for it. He fixes both these mistakes by drowning them in lava.
In Kung Fu Panda 2, if Po had not entered Lord Shen's weapon foundry against the advice of Tigress, then the Furious Five's plan to blow up Lord Shen's cannons would have succeeded without a hitch. Instead, they had to get rid of their explosives or Po would be blown up along with Shen and the Mooks.
And in the first movie Shifu, having seen Tai Lung's escape in a vision, send the goose Zeng to check and make sure Tai Lung's prison is secure. While there, Zeng accidentally drops a feather, which Tai Lung uses to pick the locks binding him and escape.
In Justice League: Doom, Batman's Crazy-Prepared status finally gets them into trouble when a team of villains decide to steal a bunch of data from his computer, specifically, the weaknesses of the entire Justice League.
Batman: Alfred, get the Batwing ready! The Justice League is under attack!
Melody, for not handing over the trident to Ariel and giving it to Morgana.
Frozen: After Elsa accidentally struck Anna with her powers, their parents kept them apart and isolated her in her room; however, being isolated and fearing her powers is the reason Elsa can't control them.
Film - Live Action
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. Nice job, hero.
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.
In Jurassic Park, 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; the raptors kill two, and relentlessly pursue the main characters for the rest of the film. Nice job, Hammond.
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, Saw VI, and arguably, Saw II) 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. Way to go, Neo.
We find from the final film, this was really a Batman Gambit by The Oracle, so Neo was filling his role exactly as intended.
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?
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 entire 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 take charge of the fusion reactor project, 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 reign 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.
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.
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 Jerk Ass 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.
Burt Gummer, resident gun nut of the Tremors franchise does this in the sequel. 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 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 attempting to free some filthy monkeys. Keep in mind 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 entire population into an 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, 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 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 3000The 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.
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.
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.
Poor Marty McFly can't go one movie without doing this trope:
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.
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 Iron Man 1 Iron Man unintentionally causes the destruction of an Air Force F-22 that attempted to intercept him (before they knew he was a good guy). In his defense, he made sure that the pilot's ejection suit worked properly. Also, he helped to design and build many of the weapons being used by the Ten Rings (and presumably the Taliban, though that much is never stated in the first film).
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.
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...
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.
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.
In X2: X-Men United, Rogue's impatience makes her indirectly responsible for Jean's death, due to crashing the Jet and causing the vertical takeoff to go temporarily offline. Furthermore, since the Jet appears to have been only about a few hundred metres from the facility, she really didn't need to fly to pick them up anyway.
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.
Charles trains Erik, helping him improve his power. This backfires when he decides to turn into Magneto.
Had Charles and Hank shown acceptance for Raven's true form, she might not have teamed up with Erik.
Magneto tries to kill Mystique because 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 needed the living specimen.
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.
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.
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.