Adventure Time: Banana Guard Academy, Peppermint Butler breaks the timeline by being unable to restrain himself from showing emotion while questioning the oracle in the basement, and then accidentally provokes an invasion of evil extra-dimensional robots by talking philosophy while the timeline is unstable.
In thisChick Tract, the protagonist's aunt, who is a conservative Christian, and thus has an interest in seeing her nephew be saved, dies and leaves him all of her money. Unfortunately, said nephew hears the news right after he was witnessed to by another Christian and seriously considering conversion. It turns out that he was only interested in Christianity because he thought it was a way out of the poverty he had stumbled into, so he decides not to accept Jesus, and that night, he dies and goes to hell.
A man proposes to a woman. The woman rejects him. Cupid fires a love arrow, only for the woman to unwittingly dodge the shot. The arrow pierces the inflatable raft the two were riding, causing them to drown.
A depressed man wanted to commit suicide. Cupid and his pals prevented all his attempts at ending his life. In his last attempt, he was going to jump off a bridge. The angels couldn't hold him back for long, so Cupid use his love arrow to make a female passerby fall in love with the man. The woman stopped the man from committing suicide and they both get married... except the woman was already married. Upon seeing this, the real husband of the woman falls into desperation and jump off the bridge.
The American space shuttle is having trouble and everyone is worried that the astronauts wont be able to repair it for their return home. Cupid and his angels friends come to the rescue and repair the shuttle, under the disbelief eyes of the astronauts. When the shuttle safely return home, the astronauts babble non-sense about angels and religion.
In one issue of Diabolik, Eva starts kidnapping people with the same blood type as Diabolik. Ginko discovers her actions, and bugs the targeted blood donor in an attempt to find Diabolik, forcing her to stop the kidnappings at 5 people. Ginko is initially proud of himself for this...then he learns that Diabolik is suffering from radiation poisoning and needs a full blood transfusion, for which Eva needs as much as 5 people have. Eva had planned to take the blood from at least 20 people, in order to avoid killing anyone, but thanks to Ginko's actions, she's forced to take all the blood from the 5 victims, resulting in 5 deaths that could have been avoided if he hadn't interfered.
In the Doctor Who Magazine strip Liberation of the Daleks, the Dalek theme park powered by psychic energy from actual Kaled mutants was probably always going to go wrong at some point. But it specifically goes wrong because the Doctor materialises in one of the simulations and, while trying to figure out what's going on, outright tells the Daleks they aren't real. And then leaves the TARDIS behind, which they can use to enter the real world.
In G.I. Joe vs. Transformers 2 the United States government decides to just nuke the heck out of the bad guys (who had all gathered on one island). Turns out nukes plus Energon, the highly-volatile substance that Transformers run on, equals Earth-Shattering Kaboom. Two heroes, Wheeljack and Mainframe and one villain, Doctor Mindbender, decide they want to live and stop the nukes by making a Kill Sat. Which then fries Mercer, a prominent good guy. Oops. The trio then has to break the Kill Sat, which fortunately doesn't cause any more innocent deaths.
Whenever Groo sets out to do good, the people he's assisting are in trouble.
The Sage tries to give good advice but it does get misinterpreted or causes more damage than good. He once told a king to build a wall to keep Groo out: this notion escalated REALLY quickly. (Worst of all, the rumors of Groo's approach turned out to be false. He wasn't in the area.)
This is the premise that kicks off I Hunt Monsters, when Willam Warlock inherits a graveyard that acts as a Sealed Evil in a Can to powerful monsters his ancestors trapped there. However, because no one told Will about this beforehand, when it comes time to recharge the obelisk keeping the monsters dormant, he refuses to believe what's going on and declines to help. Say the least, he regrets that decision a moment later when the monsters free themselves and go to wreak havoc across the world.
This happens at the very beginning of the French comic book series Les Légendaires. The Five-Man Band confronts the Big Bad and foils the plot that should grant him eternal youth... but in doing so, they shatter the magic stone he was using, which results in a supernatural discharge that turns not only the heroes but all the adults on the planet into children. (As well as the denizens of the near-by Elfin World parallel dimension.) Unfortunately for the protagonists as they struggle to correct their mistake, their responsibility in this mess is common knowledge. Needless to say, they aren't very much welcome anywhere after that.
In an Italian Disney story patterned after medieval legends, Mickey Mouse fights bravely to save Queen Minnie, showing great generosity. The more generosity he has, the more of his egoism is sent to the Moon, where Black Spot (here an evil wizard) is using it to animate an enormous army of living armours. It gets to the point that Black Spot has his Dragon Pete create chances for Mickey to be heroic, thus sending to the moon even more egoism-and, after the armours are animated, making them more powerful.
MonsterVerse: In the graphic novel Kingdom Kong, Dr. Brooks has a pretty severe case which is probably only second to the U.S. military's kill-the-Titans blunders in the films in its severity. By persisting in conducting operations on Skull Island's Vile Vortex entry, he hastens the awakening and emergence of Camazotz, which ultimately leads to the destruction of the Iwi and Skull Island's entire ecosystem by Perpetual Storm.
In the end of My Little Pony: Friendship Is Magic 5, Queen Chrysalis and her entire army escape prison because Twilight Sparkle opens the cell door and wanders in, and proceeds to get the ever-loving crap kicked out of her. Just because she wanted to give Chrysalis a book to read.
In Once & Future, Duncan's squeamishness about killing people has him stop Bridgette from shooting Galahad dead. This comes back to bite Duncan repeatedly, as it's through Galahad that Arthur has a chance to claim the Holy Grail in the first place, and Galahad quickly becomes a recurring and dangerous foe.
In The Powerpuff Girls #64 ("Monkey Business," DC run), Mojo Jojo goes straight and opens his own restaurant which is a big hit with everyone except the girls, who thinks he's up to something. After being proven wrong twice, the girls join the folks for some of Mojo's chili, but Blossom doesn't partake as she still thinks something is fishy. When she causes a panic after mistaking everyone farting for a gas leak, Mojo gives up his restaurant and goes back to crime. The narrator even chastises the girls on the end panel.
Cassie from Crossgen Publishing's Route 666 may have managed to give Congress a glimpse behind the masquerade, revealing the SovietRodinan ambassador as a literal monster and thereby striking a small blow in her fight against The Legions of Hell… but she apparently didn't anticipate that doing this at the height of the Cold War would be a perfect way to push the world right to the edge of nuclear war.
Scott Pilgrim has a very literal example of this in the fifth book: While performing at a concert live, Scott is attacked by another one of the Twins' robots. Scott deals with it easily enough; by smashing its head off with his bass guitar. Unfortunately, the bass is also destroyed in the process, which effectively puts Scott's band out of commission. Then it turns out that the bass was actually on loan from Scott's younger brother. And said younger brother comes back to reclaim the bass at the end of the book.
In Seconds, along with Katie abusing the mushroom's time space powers. It revealed that in trying to fertilize the mushrooms with the dust from the cauldron, Katie wound up bringing the house spirit of Lucknow into Seconds, who slowly starts taking it over from Lis and becoming strong enough to warp time on its own.
Locke wanted to prepare his son Knuckles for a horrible threat he sensed in a premonition. To prepare, he did genetic experiments on himself, so Knuckles would inherit the genes, and exposed Knuckles' egg to Chaos energy from the Floating Island's emerald. While this gave Knuckles more potential than any previous Guardian, said power was stolen by Mammoth Mogul, causing a chain of events that nearly destroyed the multiverse.
When Knuckles first obtained his Super Mode while searching for a spare Chaos Emerald for his island, he fought Super Sonic in a hidden zone seconds from Robotropolis. Sonic tried to stop fighting, Knuckles kept pushing things until the zone was destroyed, destroying innumerable Chaos Emeralds, and exhausting their transformations before they could use them to defeat Robotnik. The spare he did get ended up getting absorbed into the Master Emerald anyway.
The Brotherhood of Guardians had a major tradition not to interfere with anything unless it actively threatened their interests. During the last year or so of Robotnik Prime's reign of terror, he found the Floating Island, weaponized it, tried to crash it onto the Great Forest, and crashed the first Death Egg into it and tried to sink it into the ocean. Not once did they raise a finger to stop him. Even more, they didn't raise a finger to stop Enerjak, the Dark Legion, Mammoth Mogul or Dr. Eggman when he was at his weakest. Because of this, when Sonic was presumed dead, Eggman was able to catch up technologically, ally himself with the Dingos, and invade Angel Island, bringing down Echidnaopolis and allowing Dr. Finitivus to ambush and imprison the Brotherhood in the Twilight Zone.
One storyline had Dr. Eggman initiate Operation: Clean Sweep, a Cosmic Retcon that SHOULD have erased Sonic. Instead, it turned Mobius into its video game counterpart. Sonic turns Super Sonic and tries to reset things. However, because he had only barely started to remember the correct timeline, he couldn't prevent much else beyond saving Sally from being killed, doing so futzes up reality, causing a chain of events including re-allowing roboticization, restoring Ixis Naugus' other personalities, restoring Bunnie Rabbot's cyborg limbs to flesh and bone, causing Antoine to be critically injured, the Freedom Fighters to break up, and worst of all, giving Sally A Fate Worse Than Death - becoming Mecha Sally.
Mara is sent to go assassinate a gang leader, and she does, then gets rewarded for a job well done. Only to find out, months later, that she'd killed a decoy, effectively faking his death and giving him that much longer of a reign.
After the Emperor died Ysanne Isard, Director of Imperial Intelligence, pulled a Nice Job Breaking It Villain. She'd known Mara was an agent of some sort for the Emperor, but little more, and she'd been highly suspicious of her — so after the Emperor's death, Isard locked Mara up, thus turning her against the Empire.
Jedi vs. Sith: An older (timeline-wise) example. The Jedi were so desperate at the Battle of Ruusan they were giving nine and ten year old children enough Force training to be dangerous, shoving lightsabers into their hands and sending them to be cannon fodder (Seriously, are we supposed to see the Jedi as good guys?!) One of those ten year olds falls out of an airship, apparently to her death...only to survive and be found by Darth Bane. Congratulations, your child conscript is the next Sith Lord!
Tusken-raised Jedi A'Sharad Hett was shown in Star Wars: Republic to have known Anakin Skywalker personally, and he learned of his genocide of his people but refused to report him believing that he needed to confront his inner darkness himself. After Anakin fell and became Darth Vader, Hett blamed himself for not intervening. Disillusioned, Hett returned to Tatooine and became a Tusken Raider warlord only to have his right arm severed by Obi-Wan Kenobi protecting Luke's homestead. Hett was exiled from Tatooine by Kenobi, who hoped he would see the error of his ways and return to the Light. He did the exact opposite and in Star Wars: Legacy went on to become a new Sith Emperor who created the One Sith, and started a new war that resulted in the deaths of billions.
The Fel Empire (whose heroic actions tend to vary) decided to ally with the Sith and took out the Galactic Alliance so they could bring peace in the galaxy. Instead the Sith predictably back stab them and plunged the galaxy to a new war which far more destructive than before.
Cade had succeeded in killing Darth Krayt, but didn't cause a civil war among the Sith as everyone hoped. Instead Darth Krayt is resurrected and more insane than before and now plans to let the galaxy feel what he had experience in death and rebirth.
As of issue #14 of both current Transformers comics, More than Meets the Eye and Robots in Disguise, Chromedome has gotten a double-dose of this. Not only does his poor choice of words in MTMTE #14 give Overlord the motivation to break out of his prison cell and go on a rampage, but it is also revealed that his using his mnemosurgery powers to Mind Rape some incriminating memories out of Prowl's head weakened Prowl's resolve enough that Bombshell was easily able to dominate him with a cerebro-shell, leading directly to the horrible state of affairs on Cybertron that reached a boiling point in RID #14. Karma has not been kind to him, however: Overlord's first act as a free mech is to shut Chromedome inside the time-dilating prison cell where he'd been kept prisoner, ensuring that by the time Chromedome is able to escape, Overload will have had all the time in the world to murder everyone else aboard the Lost Light. Although the Autobots are eventually able to defeat Overlord, Chromedome's lover Rewind is forced to make a Heroic Sacrifice in the process.
In the same series is Whirl, before the war he was an unstable police officer who assaulted a prisoner. Said prisoner was Megatron and the experienced opened his mind to using violence overthrow the government. In The Transformers: Dark Cybertron, he's part of the reason that about seventy billion Ammonite soldiers are descending on their planet.
Whirl: A couple months back I might've accidentally ramped things up a bit. I killed their enemy's leader and one thing lead to another and now they're helping Shockwave kill us... Arcee: Somewhere in there is a lesson about unintended consequences Whirl: Speaking of which, did I ever tell you about how I started our war? You'll laugh your head off.
In the Vampirella story "... And be a Bride of Chaos" we learn that Dracula was a native of Drakulon who was supposed to be executed for his crimes, but the execution device was too powerful and instead of just disintegrating his body, it was sent into the dimension where Chaos was banished to.