Bolt: Mindy threatening to fire the director for the show getting bad ratings backfires horribly because it gives the director the idea of adding a cliffhanger making Bolt (who thinks the show is real) think Penny is in real danger leading to him getting lost in New York City while the show is forced to improvise with a replacement dog who is too afraid to do the role and accidentally sets the entire studio on fire with Penny still trapped inside.
P.T. Flea from A Bug's Life got Flik and the circus bugs ostracized by the colony and then later in the film incinerated a fake bird, thinking it was the real deal. Earlier in the film he had also fired the circus bugs, leading to them meeting Flik and setting the plot in motion.
Coraline has Wybie, who unknowingly gives Coraline the doll that allows the Other Mother to spy on her.
Not helped by the fact that most of her emotional duress (as well as Anna's social awkwardness that Hans exploits) were the result of her parents shutting them away from the world in an attempt to control her powers, when really they just hid them and never focused on helping her learn to control them.
The trolls could count as well, as most of the sister's problem stem from a bad case of Poor Communication Kills. A lot of trouble could have been avoided if they had been a little more clear. Their proclamation that fear will be Elsa's greatest enemy was accompanied by a representation of what looked like angry townspeople attacking her (an honest concern), even though they meant her fear rather than the people's (which turned out to be even more dangerous).
In the semi-canon novelization A Frozen Heart Prince Hans' plan to come to Arendelle in an attempt to marry Elsa was a plan conceived by his brother Lars. Lars, who was the only member of his family aside from their mother to treat Hans fairly well. He thought Hans should be Elsa's suitor so that he could marry into another family and have an excuse to never come back to the Southern Isles. Unfortunately for Lars, Hans let his desire to be a king, years of abuse and want for their father's respect get to him and things took the wrong turn.
In Home, during an attempted peace conference with the Gorg, Captain Smek ran and grabbed a small rock on the way out, placing it on his "shusher wand." It is later revealed that the rock actually contains the next generation of Gorg, which is the true reason the Gorg has been chasing the Boov.
Both Gringoire and Djali serve as this in the Burbank Films Australia adaptation of The Hunchback of Notre Dame - firstly, Gringoire tells Frollo where Esmeralda is going, thus allowing the priest to find and stab Phoebus, then Djali performs a trick during Esmeralda's trial, thus "proving" the accusations of witchcraft. Luckily, disaster is averted when Quasimodo rescues Esmeralda.
Anger from Inside Out frequently makes Joy and Sadness' journey back harder by making Riley overreact in ways that bring down several of the Islands of Personality, which they're trying to cross to get home. When he puts in the idea bulb to run away, not only does Honesty Island's resulting collapse bring down the Train of Thought, which Joy and Sadness were on at the time, but the idea also destabilizes Family Island before they can cross it, leaving them without any easy way home. As if that wasn't bad enough, when Anger finally sees Family Island crumbling and realizes it's a bad idea, he does so just in time for the console to lock out him, Disgust, and Fear from being able to stop Riley.
Bagheera tells Colonel Hathi about Mowgli running away, not knowing that a hidden Sher Khan is nearby listening in on the conversation, which alerts him that there is a man-caub in the jungle. A similar incident happens just before the climax.
Sher Khan overhears the vultures singing to Mowgli to cheer him up, and that draws him in to their presence. In both situations, Mowgli would have been safer if his friends had stayed silent.
In Kung Fu Panda, Shifu's goose messenger to Tai Lung's prison gives him both the means to escape and the motivation to do so. Blame is also placed on the prison commander for dislodging the feather Tai Lung used to escape.
King Triton in The Little Mermaid destroys Ariel's collection of human objects as a way of trying to teach her a lesson. Little did he know that Ursula was waiting for a moment where Ariel was vulnerable and prone to making rash decisions. Thus she's able to convince Ariel to make a deal with her - which is all part of her master plan to get control of the trident.
Nakoma in Pocahontas plays with this. She tells Kocoum about Pocahontas meeting John Smith precisely because she wants something to happen. But she was hoping Kocoum would bring Pocahontas home and stop her from fraternizing with the settlers. She certainly didn't expect Kocoum to witness a kiss, try to kill John in a blind rage and get shot by Thomas - thus leading to the near genocide of both sides. While she never gets called on her actions, she is clearly horrified by what has happened and does her best to make it up to her friend.
In The Powerpuff Girls Movie the children who ask the girls to play tag with them on the first day. This leads to the girls getting carried away with their powers and causing massive amounts of destruction to the city. Thus Professor Utonium is held responsible and thrown in jail, allowing Mojo Jojo to convince the girls to side with him and nearly enslave the city.
In the first, her insistence that Andy can only bring one toy with him to Pizza Planet ends up causing Woody, knowing Andy will pick Buzz over him, to accidentally knock him out of a window, leading to both of them getting lost.
In the second, her decision to sell Wheezy at the yard sale leads to Woody going out to the sale to save him, and Woody ends up getting stolen by Al.
In the third, she accidentally brings the toys out to be thrown away, even though Andy intended to store them in the attic. The miscommunication causes them all, besides Woody, to donate themselves to Sunnyside.
Hot Rod was a borderline The Load during the fight between Optimus Prime and Megatron in The Transformers: The Movie. He promptly was hostage by Megatron, resulting in Optimus' death.
Wreck-It Ralph: If Gene hadn't goaded Ralph into getting a medal, many, many problems could have been avoided. On the other hand, no one would have discovered a dark secret about Sugar Rush either.
In The Amazing Spider Man 2, Spidey manages to calm Max Dillon down and is ready to talk it out with him without any fisticuffs, until a police sniper interpreted Max suffering from Power Incontinence as a hostile act and shoots him. This and the New Yorkers rallying behind Spider-Man while heckling Max caused him to further go through Sanity Slippage and start to believe Spider-Man betrayed him.
Snoop, the airport sniffer dog from Babe: Pig in the City falsely detects that Babe and Esme are carrying illegal substances. Thanks to the interrogation that he brought upon them they miss their flight and are forced to stay in Metropolis. It is here where the film's plot really starts to kick in.
In Back to the Future Part II, a man from 2015 makes an offhand comment to Marty McFly about using knowledge of the future to bet on sports games. The end result is the Biff-horrific 1985 that Doc Brown and Marty return to.
In Akira Kurosawa's The Bad Sleep Well, Nishi's revenge plot is scuppered by his father's well-meaning widow, who innocently reveals his real identity to one of the men Nishi wants to destroy.
Linda in Burn After Reading. She sends Chad to Harry's house where he winds up dead, then sends Ted over to find what happened to Chad (who then ALSO ends up dead), then accidentally scares Harry into fleeing the country when she mentions Chad's disappearance in a conversation with him.
In The Elite Squad, there's the unknown journalist who takes a photo of Matias and inadvertently reveals the fact that he's a cop to the drug dealers. It ruins Matias' relationship and leads to another character's death.
Kenny Linder from The Fast and the Furious. If he hadn't accidentally killed Dominic's and Mia's father in a racing crash, Dominic wouldn't have beaten him savagely with a wrench and gotten himself banned from racing. And then he wouldn't have turned to hijacking trucks to fund his street racing. And then Brian wouldn't have gotten involved, etc. In short, Kenny Linder is indirectly responsible for the entire series.
In The Fisher King, radio shock-jock Jack Lucas isn't aware that one of his callers, Edwin, is mentally unstable when he sarcastically tells Edwin to go kill some yuppies. This inspires Edwin to commit a mass murder-suicide at a Manhattan restaurant, which not only costs Jack his career and turns him suicidal, but destroys the sanity of one victim's widower to the point he becomes convinced that he is a knight seeking the Holy Grail.
It! The Terror from Beyond Space: Calder dumped some empty crates and other assorted trash on Mars and forgot to close the rocket's airlock up until it was almost time to launch — which was a long enough time for the alien that killed (almost) every member of the Challenger 141 to get inside the Challenger 142 and continue its rampage with fresh victims.
In Jaws, if Mr. Kintner did not allow his son Alex to return into water for a last time before returning home, Alex would not have been killed by a great white shark in a gruesome pool of blood.
In The Lord of the Rings: The Two Towers, at least in the film version, Faramir is one. He is prepared to shoot and kill Sméagol for entering the Forbidden Pool and seemingly allows Frodo to call him out of the pool, but immediately captures him upon his exit. Sméagol thinks Frodo lied to him and, enraged by this apparent betrayal, regresses into his Gollum persona and starts plotting his and Sam's deaths for the sake of retrieving the Ring. In the director's cut, the honor goes to Sam: he spends the whole movie antagonizing Sméagol, so when he tries to explain to him at the end that his capture was not Frodo's decision, Sméagol is quite obviously unconvinced.
Ultimately, however, this is subverted- in both the book and the movie, Sméagol's survival and later interference at Mount Doom is the only reason the One Ring ever gets destroyed at all. If Faramir had ended him back then, Frodo would have taken the Ring for himself- for as long as that might have lasted. As depicted in both the book and the film, the moment Frodo put the Ring on was the moment Sauron and the Nazgûl knew where he was; and as they can see him even when with the Ring on, they'd have killed him and delivered it to the Dark Lord himself. In the end, the mercy of Bilbo, Frodo and Faramir ends up determining the fate of Middle-Earth.
In Avengers: Age of Ultron, Scarlet Witch is so hellbent on getting back at Tony for creating weapons that killed her parents that she mind rapes him with a nightmare vision that shows all of his friends and allies dead and Earth taken over by aliens. Terrifed that the Avengers would not always be able to succeed, Tony creates Ultron to help with peacekeeping. As soon as he does though, Ultron quickly turns against him. This results in Sokovia's destruction, her brother Quicksilver being killed in action and, much later, the fall out of the Avengers.
In Guardians of the Galaxy Vol. 2 the Ravagers are enraged when Yondu chooses a lesser payday rather than betray Quill. Kraglin chooses this time to vent his frustration about Yondu playing favorites, resulting in a mutiny that sees all of Kraglin's friends spaced while he watches.
Kraglin: I didn't mean to do a mutiny. They killed all my friends.
In Black Panther (2018), T'Chaka, then the Black Panther, confronts his brother N'Jobu about his involvement with Klaue's theft of the vibranium, which T'Chaka learned about from his informant Zuri. When N'Jobu tries to kill Zuri, T'Chaka is then forced to kill N'Jobu. Both T'Chaka and Zuri then return to Wakanda, leaving behind N'Jobu's orphaned son Erik... who eventually becomes the villain Killmonger, who seeks to achieve what his father set out to do.
So, so many in Avengers: Infinity War. Among them: Earth wouldn't have been so vulnerable to Thanos if Ross hadn't torn the Avengers apart; Thanos wouldn't have the Time Stone - or indeed, the Gauntlet - if Star-Lord hadn't ruined his own plan in a fit of grief and rage after discovering that Thanos killed Gamora; and Gamora wouldn't be dead in the first place if Nebula hadn't tried to kill Thanos, causing a chain of events that leads to Gamora being sacrificed for the Soul Stone.
Evey in The Mummy (1999) merely was curious about what was in the Book of the Dead. Little did she know that reading aloud from the first page would resurrect Imhotep and bring about the ten plagues of Egypt. This is lampshaded in the sequel where Evey wants to open a sacred chest and Rick reminds her of what happened when she read the book.
In A Nightmare on Elm Street 4: The Dream Master, Elaine Parker not only was responsible for her daughter Kristen's death (she put sleeping pills in Kristen's drink), but thanks to that happening it gave Freddy the opportunity he needed to successfully establish a link to the rest of Springwood's teen populace. So basically every death from this movie onward is because of her. Nice job, Elaine.
The parents of the Elm Street series are this as a whole, as are the cops. Everyone was so desperate in catching the child killer that someone forgot to sign the warrant for his arrest, leading Freddy to be released from prosecution almost immediately. Enraged, the parents chased Freddy to the boiler chamber where he killed his victims and set the place on fire with him still inside. To their credit, they had no idea that doing that would turn him into something much worse than a child-killer.
Cadet Fackler from Police Academy is given an apple by his partner while they're on patrol. He tosses it over his shoulder and hits a tough guy in the back of the head. This starts a chain reaction which eventually results in a riot. In the following scene, he tells Hooks and Thompson about the riot. He firmly establishes his status as this when they ask him what caused the riot and he replies "Who knows how these things get started?"
The first Pumpkinhead movie's chain of events is started by Bunt Wallace, a backwoods teen who offers to tell Ed where to find the witch that can summon the eponymous demon simply because he wants to see if it's actually real. When he learns the full scope of what he'd done, he's rightly horrified.
In Robin Hood: Prince of Thieves, his blind manservant Duncan gallops into Sherwood Forest in order to tell his master that Marian has been captured by the Sheriff; only for the bad guys to follow him straight to the outlaws' hideaway and start to massacre everyone there. Beyond getting ushered about by either Robin or Marian, it's the only thing he ever does in the movie.
Craig and Mary-Ann, the two teens in the opening who accidentally cause the boating accident that kills most of the Baker family and kicks off the entire franchise.
A less innocent example would be Aunt Martha. True, her forcing Peter to adopt the identity of his dead sister was horrifying and abusive but she probably didn't expect it to escalate to serial murder.
In A New Hope, the Star Destroyer officer who orders his subordinate not to fire on the escape pod containing C-3PO and R2-D2, thus ensuring that Luke Skywalker gets Princess Leia's message and brings about the end of the Empire. If this nameless officer had not been so frugal with laser ammo, The Dark Side would surely have triumphed.
In Attack of the Clones, the one thing that Jar Jar actually does in the movie, other than stand in the background, is to make a motion in the Senate to grant Palpatine emergency powers. Yes, thatPalpatine. Jar Jar was already so despised at this point that the reaction was mostly along the lines of "He was the cause of all the evil in the universe? I knew it!"
If Admiral Ozzel in The Empire Strikes Back hadn't been so impetuous and exited hyperspace too close to Hoth, the Rebels wouldn't have had time to set up the shield generator and the Imperial fleet could have just bombed them from orbit. This leads to the trope namer for You Have Failed Me.
Wicket in Return of the Jedi is the only reason the Rebels won. If he didn't pull a Big Damn Heroes moment by bringing an army of Ewoks to fight the Imperials, the Rebels would never have destroyed the shield generator and never would have destroyed the second Death Star. The Rebels only met the Ewoks at all because of a Scout Trooper who knocked Leia off of her speeder bike, leading to her befriending them while the others searched for her. If he had aimed a little higher, none of the above would have happened (how appropriate that the Empire's downfall is ultimately caused by the Imperial Stormtrooper Marksmanship Academy).
Anakin is the Instigator of his own doom. If he hadn't finished that protocol droid he had started in The Phantom Menace, things could have been very different. One possible unfolding: R2-D2 would have never been sold to the Lars family without C-3P0's endorsement, thus R2 stays with the Jawas, R2 gets vaporized when the detachment of Stormtroopers find the sandcrawler, and Leia's message never reaches Obi-Wan.
The Shirley MacLaine comedy What a Way to Go! could alternately have been titled Unwitting Instigator Of Doom: The Movie, with MacLaine's character being widowed four times after her entirely well-meaning suggestions inevitably, relentlessly, snowball into her current husband getting killed in some bizarre fashion.
In Wrong Turn 4: Bloody Beginnings, a prequel to the Wrong Turn series, one of the characters beg someone not to kill the inbreeds. They spare their life, but in return are responsible for nearly every single death in the films.
X-Men: Days of Future Past: Mystique's plan was simply to avenge a number of dead mutants by murdering the man who had abducted them and experimented with their corpses. That man had projects, rejected by the Congress, to build powerful robots to kill mutants. His death proved his point, that mutants were an actual menace, and so his projects were restored and continued. The Sentinels would prove so deadly and effective that they would cause the apocalyptic future seen at the beginning of the story. Of course, Mystique had never intended any of that; all she wanted was plain revenge.
X-Men: Apocalypse: After mutants became public knowledge in 1973, a cult was formed to worship En Sabah Nur, and Agent Moira MacTaggert's investigation inadvertently awakens him because his followers always cover up the entrance to his resting place, but she had left it exposed to sunlight, which reanimates him.
Lieutenant Gorman's last act in Aliens indirectly causes the events of Alien³. The blast from the grenade he uses to commit suicide with causes Newt to fall into the flooded section of the colony where she gets abducted by the aliens. This forces Ripley to rescue her from the atmosphere processor, giving the Queen alien a chance to stow away to the Sulaco. The eggs the Queen managed lay aboard ship eventually kill everyone in Alien 3.
A Simple Favor: Emily winds up murdering her twin sister and faking her own death so she and her family can get the insurance money that would come from it — four million dollars. This would be enough to get the family out of debt, and solve a lot of problems... but, needless to say, things quickly go haywire, kickstarting all the drama in the main plot. And the cause of all this trouble? A perfectly innocent comment by Emily's friend Stephanie, who says that she's relieved her husband had life insurance, because otherwise she wouldn't have been able to care for her son after he died.