Heroes aren't perfect and that's definitely true at Marvel. The heroes of the Marvel Universe have always been portrayed as more realistic than others and they have made a multitude of mistakes during their time as heroes. Sometimes, these mistakes have been rather easily overcome but other times, they've led to some terrible consequences not just for the heroes but also for the world in general.
Even in the pantheon of bad mistakes, superheroes can make very costly errors. They have great responsibilities and when they miscalculate, it leads to grave consequences.
10 Hank Pym Trying To Destroy Ultron Caused Untold Destruction
Hank Pym is one of the Avengers' oldest members but he's also screwed up a lot. The worst of those involve his creation of Ultron. While one can look at his making Ultron as his biggest mistake, the worst mistake is the fact that he tried to destroy Ultron when he realized that the android was self-aware. It was this act that began Ultron's hatred of his father and humanity.
Ultron looked at himself as Pym's son, so his father trying to kill him broke his mind. If Pym wouldn't have done that, Ultron may have ended up joining the team and becoming a robotic superhero instead of a mass-murdering android.
9 The Illuminati Going To The Skrull Homeworld Sowed The Seeds For The Eventual Skrull Invasion
The Illuminati got up to some rather shady things in their time but one of the worst took place after the Kree/Skrull War. The group went to the Skrull homeworld to warn them about attacking the Earth again. This was such a hubristic thing to do and it backfired totally. The Skrulls took the group hostage and began to experiment on them.
While the team was able to escape, the Skrulls were able to use what they learned from them to master giving Skrulls powers, which allowed them to attack the heroes in Secret Invasion and almost conquer the Earth.
8 Uncanny X-Force Didn't Notice Archangel Becoming Apocalypse, Which Cost Thousands Of Lives
Wolverine's Uncanny X-Force, consisting of Psylocke, Deadpool, Fantomex, and Archangel, went after Apocalypse's newest clone in their first mission together. This mission planted the seeds that almost destroyed the team. It awakened something in Archangel, and his Apocalypse-created dark side would take control.
The team didn't notice this at all and it eventually cost thousands of lives. Working with Genocide, Dark Beast, and the Final Horsemen, Archangel destroyed the entire town of Tucker Creek, killing thousands. If X-Force had been more diligent, they could have prevented that.
7 Black Bolt Releasing The Terrigen Mists Led To The End Of His Kingdom
The Inhumans have done a lot of bad things but one of the worst was when Black Bolt released the Terrigen Mist clouds. This had wide-ranging repercussions. It awakened many Inhumans around the globe, which was a plus for the race, but that was the only positive. It also pushed the mutant race to the brink of extinction, a move that led directly to the Inhumans' downfall.
The two groups had a battle that weakened the Inhumans and shattered their unity. This led to the Royal Family being eventually killed, as Attilan had fallen apart after the war against the mutants.
6 The X-Men Trusting Sabretooth Led To Psylocke Almost Being Killed
Sabretooth is a sadist of the first order, a lesson that the X-Men have apparently had to learn multiple times. One of the most damaging to the group came after Wolverine lobotomized Sabretooth by popping a claw into the villain's brain. After this, Sabretooth pretended to basically be a brain-damaged kitten and X-Force member Boom Boom got closer and closer with him.
This led to Sabretooth attacking her in a bid to escape the X-Mansion. Luckily for Boom Boom, Psylocke was there and attacked the feral mutant. Unfortunately, Sabretooth was just entirely too much for Psylocke and almost killed her.
5 Anyone Trusting Iron Man Ever Is A Massive Mistake
Iron Man is one of the shadiest heroes in the Marvel Universe. While it's impossible to deny that he's done a lot of good for the world and been a great boon to the superhero community, actually trusting him is a huge mistake. For example, he combed Avengers Mansion in order to get one of Thor's hairs so he could clone him, which would eventually lead to the creation of Ragnarok, who killed Bill Foster. That's just the beginning of the betrayals of his fellow heroes.
Iron Man is just inherently untrustworthy. He believes he's always right and will do whatever to make sure that he gets his way. He's done horrible things, hurt his closest friends, and basically done as much damage to the heroes as any villain.
4 Professor X Mindwiping Magneto Led To Him Becoming Onslaught
Magneto ripping out Wolverine's adamantium was an event with wide-ranging consequences. One of the biggest was Professor X mindwiping Magneto. This may have stopped Magneto's threat for a while but it also started Xavier down the darkest road. A piece of Magneto had bonded itself with Xavier's mind and formed the entity Onslaught.
Onslaught would go on to attack the heroes of the Marvel Universe, nearly destroying the X-Men before using Sentinels to attack New York City. This attack led to the human heroes being shunted to an alternate dimension and cost many lives, all because Xavier lost his cool with Magneto.
3 Captain America Joining The Illuminati Was Completely Futile & Ended In Him Getting Mindwiped
The Illuminati have been responsible for a lot of terrible things but one of the worst was their betrayal of Captain America. Cap joined the group to inject his own brand of morality into it but he wasn't ready for the kind of things the team was going to have him do. His joining them coincided with the Incursions, which led to the group preparing to commit genocide on an unimaginable scale.
Of course, Cap was against their decision and it led to Iron Man having Doctor Strange mindwipe Cap. Eventually, Cap would find out what happened to him as a member of the Illuminati, which would hurt his relationship with several heroes.
2 Spider-Man Revealing His Secret Identity On TV Was A Huge Mistake
Like many heroes, Spider-Man made the mistake of trusting Iron Man and it almost cost him dearly. In the wake of the Superhero Registration Act's passage, Spider-Man listened to the Armored Avenger and went on TV, revealing his secret identity to the world. Eventually, Spider-Man soured on Stark's actions in the superhero civil war and switched sides, with his family going into hiding so Stark couldn't use them against him.
Unfortunately, Kingpin was able to find them, and Aunt May was shot. This led to Spider-Man and Mary Jane trading their marriage to Mephisto, which led to the infamous story One More Day. All of this would have been avoided if Spider-Man hadn't made the mistake of revealing his identity.
1 Wolverine Not Killing Magneto Quickly Led To Him Losing His Adamantium
Wolverine is one of the most dangerous mutants on the planet but him stopping to gloat cost him and the world dearly. Wolverine was part of the strike force that went to deal with Magneto in X-Men #25 and successfully got the drop on the evil mutant, almost killing him with his first blow. Unfortunately, he stopped to gloat which gave Magneto the time to recover and attack him.
Magneto ripped out Wolverine's adamantium, which led to Xavier's mindwipe of Magneto, which led to Onslaught. All of this could have been prevented if Wolverine hadn't stopped to gloat over Magneto's prone body and finished the job he started.