Done on a universal scale by Reed Richards. Reed learned some time ago that Galactus' existence, however problematic, is necessary to the universe. He tried to get around the problem by turning Galactus into a star. In mainstream continuity, this still ends up releasing Abraxas. In Earth X, it frees up the Celestials to overrun the universe (because what Galactus was really eating was their young, which gestate in planetary cores — then hatch).
Done a lot in the Byrne Era. The only characters who don't mess up royally are Sue and Johnny. For example: Reed invents a portal to the Negative Zone. He then takes Sue, Johnny, and Ben to the Negative Zone for a long time without even considering the fact he didn't even program the portal to only recognize them, and not allow anyone else in. Oh, not only that Reed never once considers asking the Avengers to watch over Alicia and Franklin in the Baxter Building, leaving them completely and utterly defenseless in case of an attack. The result? Alicia and Franklin are nearly killed by Annihilus, half of the Baxter Building is destroyed, and Annihilus almost destroys New York yet again. And...no one calls out Reed that stupidity. Not even Sue!
Another point: Thing decides, against all logic, to stay on the planet where Secret Wars takes place. He returns months later, learning Alicia had moved on and fell for Johnny (which was mutual). Ben, we love you, but come on, you ABANDONED your blind girlfriend after she had been fucked up by Annihilus to stay on another planet due to your self-centered body issues. And you expected her notto move on? Granted, "Alicia" was really the Skrull Lyja, but even so...
In Jenny Sparks: The Secret History of The Authority, we find out that a thirteen-year-old Jenny Sparks convinced a friend in Vienna that he was wasting his life making paintings that didn't sell and suggested that he'd find another profession.
Jenny Sparks: There must be something you can do. You're patriotic, well-read and an excellent communicator. Have you ever considered a career in local government? Do I even have to say it?: Politics? Actually, that might not be such a bad idea.
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 early Avengers issue, the team has come to a military base to locate the Cosmic Cube, an all-powerful wish-granting machine. They find Prince Namor the Sub-Mariner trashing the place for unrelated reasons. During the fight, Hercules tells Namor they'll never let him get the Cosmic Cube. Namor promptly escapes, leaving Hank Pym fuming — he's guessed (correctly) that Namor had never heard of the thing until Herc told him. Naturally, Subby finds the Cube and comes back to mop the floor with them.
Ah, the Guardians of the Universe. Living proof that age does not necessarily equal wisdom, as it seems that in one way or another, pretty much every single one of their decisions backfires spectacularly. Highlights of their past decisions coming back to bite them in the ass include the roboticManhunters, rogue Guardian Krona, rival corps such as the Red Lanterns and the Sinestro corps, and, last but not least, the events of Emerald Twilight. And that was beforethey went off the deep end. It is not an exaggeration to say that most of the conflict from the GL books stems in some way from the Guardians. No matter what decision they make, action or non-action, it always ends up being the wrong one.
Another Green Lantern example: When the Corps executed Sinestro in Green Lantern Corps vol. 1 #222, Sinestro transferred his soul to the Central Power Battery and imploded it from within (rendering almost all of the power rings powerless, with the exceptions of Hal, Guy and Ch'p), and that's when he discovered the yellow impurity (if we go by current continuity, that yellow impurity is Parallax).
Guy Gardner, in Green Lantern Corps #16, "Falling Star", botches a sting operation that was 2 years in planning by the Feds and local police, when he pulls a Leeroy Jenkins and crashes the setup, sending all their hard work down the crapper and getting himself incarcerated in the process.
Supergirl: Supergirl did this twice in Red Daughter of Krypton. In Red Lanterns #31, Supergirl tries help a Red Lantern get her mind back by taking a tank of lake blood and ripping it open over the city that Sheko -the above-mentioned Red Lantern- is in. Unfortunately, Atrocitus is present, and he takes control of the blood rain, making it into a tornado. So Supergirl tries to fix the mess. Unfortunately her way to fix the blood storm is to ignite it with heat vision. The result conflagration sets the city on fire.
War World: In order to save his friends from villain Mongul, Superman fights Martian Manhunter and retrieves s special Key from a crypt. Later he refuses to hand over the device, but Mongul manages to get his hands on the Crystal Key and start Warworld, a satellite super-weapon.
In Booster Gold vol. 2, when he goes back in time to save his friend Ted Kord, the second Blue Beetle. He succeeds, but since Ted's death is the nudge that brought the other heroes into action barely on time for the events of Infinite Crisis, his salvation results in the heroes failing to act on time, leaving the world in ruins and under Max Lord's domination.
Almost and sort of happened in the Rock of Ages storyline. As Superman and the other mainstringers of the JLA go after Lex Luthor, who has acquired the Philosopher's Stone and formed his own Injustice Gang, it is revealed to Green Lantern, Flash, and Aquaman that the JLA will eventually defeat Luthor and the Gang, and Superman will destroy the powerful Stone… which will somehow cause Darkseid to conquer the Earth. They even get a firsthand experience of that dark (pun not intended) future. This future is averted when they get back barely in time to give Martian Manhunter a warning for him to stop Superman from destroying the Philosopher's Stone.
Definitely happened in the Midsummer's Nightmare storyline (which caused that JLA incarnation to form in the first place): after foiling the Big Bad's plan, he explains he only did it to prepare humanity for a worse threat Mageddon that was still to come.
In their battle with the invincible supervillain the General, Batman tries to use hypnosis to defeat the villain. He does a good job until Superman comes bursting through the wall. Batman even yelled at Supes for the screw-up.
Sleepwalker has just about defeated the ax crazy Psyko and halted his rampage across New York. It's then that he sees a man who's possessed by a demon and uses his warp beams on the man to expel the demon, which provokes a watching Spectra to attack him. While Sleepwalker defends himself from Spectra, Psyko seizes the opportunity to escape and continue running amuck through New York. Fortunately, Spectra later fixes her mistake when she helps Sleepwalker bring Psyko to justice once and for all.
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.
Raven screwed up big time in Teen Titans. During the one-year Time Skip (long story), she spent several months working on bringing her old friend, Joseph Wilson, back to life. She succeeded, and what was the first big thing Joey did after returning from the grave? Tried to murder the Titans. Thanks, Raven. What would we do without you?
Another Teen Titans example is when Damian joins the team. The Titans are fighting a rampaging teenage psychic. Raven is able to calm him down. Then the Boy Wonder kicks him, pissing him off even more.
Also happens in Marvel's Crisis CrossoverSiege. The Avengers assemble in Asgard to defeat Norman Osborn, reuniting the 'Big Three' of Marvel; that is, Captain America, Iron Man and Thor. Osborn is defeated, but not before having The Sentry destroy Asgard. It then turns out that Osborn was the only one who could control the Void, the Sentry's evil side, who, as the president was informed, "has no limit to his powerset". The Void is then shown preparing to kill them all.
In a six-issue miniseries about Mara Jade, By The Emperor's Hand, 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.
For that matter, 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.
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!
The Fel Empire in Legacy (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 predictable 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.
At least two or three of his Rogues Gallery have been created this way as well, mainly by accidentally causing their downfall (at least one of The Joker's Multiple-Choice Past stories have him being scared by Batman and then either falling or jumping into the chemicals that result in his current look, and in some adaptations of The Riddler's backstory, it's implied he once worked as a game programmer before Bruce Wayne fired him), both in the comics as well as in various adaptations of the comics. Nice job breaking it, Dark Knight.
The events of Death of the Family are this: Bruce Wayne, early in his career, confronted the Joker after he's first incarcerated and makes the mistake of giving a hint of who he is to the Clown Prince. Cue a few years later in-series, and the Joker decides to destroy Batman's family and bring everything back to square one. Oy, Bats...!
Being Crazy-Prepared, Batman has a marked tendency to create draconian plans in case he ever needs to, say, destroy the Justice League or trigger a massive gang war. Unfortunately, he is also terrible about securing said plans. Thus, it is basically inevitable that someone will end up stealing these plans and using them against his allies. The first time this happened, Ra's Al Ghul nearly killed the Justice League, and he turned out to be a trendsetter.
Then, there's War Games. Stephanie Brown, pissed off at being used by Bruce, decides to use one of Bats' contingency plans to gather up all of the gangs. However, she had no idea that Batman was actually needed for this part. End result? All of the gangs killing themselves, leading to a massive power vacuum that ends up with Black Mask in charge.
When a minor villain named Cluemaster (a Riddler expy) was captured and sent to prison, it did cure him... of his obsession to leave clues.
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.
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.
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.
Barry Allen decides that he doesn't want his mom to be dead anymore. So, he goes and chases Professor Zoom and stops him from killing his mom. End result? Flashpoint Oops.
He did this literally, on a smaller scale, in one issue of his original run. The Mirror Master falls in love with a woman trapped in a mirror, and tries to rescue her by tricking the Flash into taking her place. Barry realizes he's being sucked into the mirror—and shatters it, which traps her forever. Oops.
The Mighty Thor: In-universe, almost everyone considers Thor bringing back Loki (after he died in Siege) to be this. Loki was, however, brought back as a kid without any of his former emotional baggage (this was discovered to be one last shot at redemption/something like that by the original Loki). The trope's been subverted because so far the kid has one goal in mind: save Thor from the Serpent (and he if saves Earth too, then that's great!).
Plus, Thor insists that the real Nice Job Breaking It, Hero! was how all of Asgard treated Loki as a villain because that's how it was written by the Norns, and therefore he became the villain they scorned him as. The main argument Thor has for keeping Loki alive is that with the Ragnarok cycle broken, even Loki has a chance to be good, and not have his fate predetermined.
Odin seems to be setting himself up for one, with the whole "raze earth to the ground to kill one enemy" thing. That won't come back and bite you in the butt ever. After all, the citizens ofMarvel comics are so forgiving.
They're only unforgiving if you're a good guy. Bad guys says he's reformed? Let's put him in charge of S.H.I.E.L.D.! Nice job breaking it, Muggles.
One Archie Comics' Sonic the Hedgehog 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.
Long before this, 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.
Hell, it runs in the family! 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.
Avengers vs. X-Men: The Avengers manage to disrupt the Phoenix Force before it can possess Hope... only to split it into five units, which then possess Cyclops, Namor, Emma Frost, Colossus (who was already possessed by the Juggernaut!) and Magik. The Phoenix-possessed X-Men then proceed to use their new power for the greater benefit of all mankind, solving world hunger, making deserts bloom, bringing free clean energy to every corner of the globe and even bringing about an end to war... then the Avengers decide to attack them again.
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.
Justice League: The Rise of Arsenal became less about Roy Harper's downward spiral in anti-heroism following the death of his daughter Lian and his dismemberment by Prometheus, and more about what happens when you neglect a friend when he needs you most. This was due to the astonishingly half-assed approach his friends and family took when trying to help him.
Kid Loki is directly to blame for all the final conflict in his run of Journey into Mystery. Actually, Kid Loki's entire run is a breathtaking example of this trope in action. Mostly powerless and mistrusted by nearly everyone, Loki's only recourse to solving the problems he encounters is to knowingly and willfully break other things. The final conflict is, essentially, everything he's done catching up to him.
In Matt Fraction's 2011 The Defenders series, the heroes accidentally meddled with one of the Concordance Engines — mysterious artifacts that keep the 616 universe running as it should — in the very first arc. The rest of the series involved them learning how badly they'd messed everything up, and eventually Doctor Strange had to go back in time and retcon their mistake (and the whole series) out of existence.
In the end of Fiendship 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.
Happens on a major scale in the Marvel crossover event Age of Ultron, when Wolverine's jumps through time tear open the fabric of space and time, creating a multiverse crisis.
The Skrull prince Xavin put a great deal of effort into guilt-tripping Karolina Dean into leaving the Runaways and becoming his/her bride in hopes of ending a war between the Skrulls and the Majesdanians that had been sparked by Karolina's evil parents. But their betrothal actually ended up re-igniting the war and making things worse.
For someone calling himself the Superior Spider-Man, he's sure doing a great job messing things up. Let's see here:
He actively attacks The Avengers when they worry about him, then put him on probation when they can't really find anything wrong and suggest he's just taking things a tad too far.
After obtaining his own mercenary force and obtaining the Raft to turn into Spider-Island, he invades Shadowland and apparently kills Wilson Fisk. Not only is Fisk not dead, the Green Goblin swoops in and gathers up what remains of Fisk's forces to become the Goblin Kingpin of Crime.
When he's searching for the new Hobgoblin, he goes so far as to reveal the man's identity to the entirety of New York. With nowhere left to run, the man finds himself running into the loving arms of, you guessed it, the Green Goblin.
His ultimate breaking? Erasing Peter Parker's memories from his mind. In his attempt to prove that he's better than Parker in every way, he eradicates what seemingly is what's left of Peter's memories. End result? When Horizon Labs is on the verge of being destroyed and another hero, Spider-Man 2099 attempts to save the day, SpOck promptly knocks him out, then attempts to save the day himself. Because he erased Peter's memories, he's trying desperately to find one of Peter's memories before he's blown up temporarily and, thanks to a bit of a time glitch, loses about nine hours of life. Thanks to this, Horizon Labs is gone, he's lost his job, Spidey 2099's stuck in the past... oh, and Peter's coming back.
Not that Parker himself has done much better. His 'solution' to the Goblin Nation mess was to cure Norman Osborn's insanity. While this might seem like a good idea, it's obvious that without Green Goblin holding him back, Normie is going to be a far more cunning and insidious super-villain than the cackling lunatic he used to be.
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.
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.
Nick Fury decided to go after Latverian terrorists in the Secret War crossover. But he didn't get the job done all the way, and the entire thing ended up biting him in the ass, with Luke Cage and Jessica Jones in particular almost getting killed, and Nick on the run for his life from the very organization he had led for years.
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.
In Swordquest: Waterworld, Captain Frost kills a snow-whale to prevent them from siding with the local mer-people in a possible war, but that's the very atrocity that prompts them to immediately declare war.
In Convergence: Superman #1, the pre-Flashpoint Superman's attempts to reason with Captain Thunder and the Flashpoint versions of Cyborg and Abin Sur are foiled when the pre-Flashpoint Jimmy Olsen, wanting to help Superman, attacks the Flashpoint heroes with an armed aircraft.
In Convergence #7, Parallax kills Deimos but this let loose of all the powers Deimos had drained from the time travellers to BREAK all of reality apart.
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.
In Emily Carroll's "A Lady's Hands are Cold," from Through the Woods, the protagonist, a young woman in an Arranged Marriage, finds pieces of the murdered first wife concealed all over the house. Wife #2 reassembles Wife #1, which appears to be what the corpse wants. Unfortunately, now that she's reassembled, Wife #1's first goal is to kill Wife #2.
In Ultimate Fantastic Four, Reed Richards created the Cosmic Cube to defend Earth against Thanos... which is exactly what Thanos was manipulating him to do.
In one of the tie-ins to Original Sin, it's revealed that during World War II, The Invaders had a chance to defeat Imperial Japan without dropping the atomic bombs, but chose not to go through with it. The explanation given is that Golden Girl and the Kid Commandos purposefully sabotaged the plan, as it would've endangered the lives of a bunch of native islanders living near the Japanese coast. The plan's failure is what led to President Harry Truman's decision to bomb Hiroshima and Nagasaki, resulting in far more civilian deaths than would have occurred had Golden Girl not interfered. Radiance, Golden Girl's granddaughter, understandably goes berserk when she learns this.
Joker's Last Laugh would not have happened if a doctor hadn't decided that lying to the Joker and tell him he has cancer to screw with him was a good idea.
War of Kings: After the events of Secret Invasion, the Inhumans decided they had taken enough being pushed around, and go after the Skrulls. Their first and only strike, using Black Bolt's voice, utterly destroys the ragged Skrull fleet... and punches a hole in reality, awakening the long-dormant Fraternity of Raptors (a Magitek Shi'ar cult of assassins). It also provokes Vulcan, who takes it as an excuse to invade Kree space to try and kill them, kickstarting the whole war. Later on during the war, said Fraternity assassinates Lilandra, just as she was trying to depose Vulcan and regain control of the Shi'ar, to try and end the war.
Amazingly, it does not get better. At the conclusion, Black Bolt and Vulcan get into a fight which tears open a massive Negative Space Wedgie named The Fault. Adam Warlock, of the Guardians of the Galaxy, goes to fix it, but his methods not only cause the return of The Magus, it begins violently overriding every possible future with one ruled by the Universal Church of Truth. The Guardians manage to prevent that, but the Magus uses the Fault to summon the Many-Angled Ones to take over the universe. That eventually results in Star-Lord and Richard Rider having to sacrifice themselves. All because of Black Bolt.
Ultimates 2015: As part of their team mission, the Ultimates try to travel beyond the edge of all existence, to try and "fix" the broken timeline. It doesn't work, partly because other parties persuade them to go home. But the dimensional hole they punch draws the spirit of Thanos back to the world of the living, which causes the events of Civil War II.
In Part 4 of Night of the Monster Men, Gotham Girl kills a monster by ripping its arms off. However, this drenches her and Nightwing in its blood and causes them to mutate into monsters themselves.
During Civil War II, Hawkeye killed Bruce Banner. It is revealed that Bruce had asked Hawkeye to kill him if it looked like he would ever again become the Hulk, and had even given the archer a special arrow tip to do the deed with. As a result, the Hand was able to acquire Bruce's body and turn the Hulk into a brainwashed servant of theirs, and Jennifer Walters has another bit of trauma and anger to deal with. Meaning that she's becoming as savage as the Hulk was. Um...oops?
Cable managed to kill En Sabah Nur in the 12th century, before he could become Apocalypse. Later, he learns that the blood he spilled in that fight mingled with Nur's and resurrected him, made him techno-organic, and gave him the ability to interface with Celestial technology, effectively creating Apocalypse in the first place.