Batman is undoubtedly DC Comics' most popular and beloved character. He is the Dark Knight Detective, the Caped Crusader, and Vengeance personified. Batman is a central figure in the Justice League and organized the Gotham Knights, Batman Incorporated, and the Outsiders.
Batman has a genius-level intellect and only wants what's best for his city, but that doesn't make him immune to mistakes. Unfortunately, a lot of people rely on Batman to not make mistakes, so, when Bruce Wayne falters, bad things happen to the surrounding people. Characters can and do die. Batman has made some pretty significant mistakes over the years, and the results can get pretty grisly.
8 Not Believing In The Court Of Owls
As one of the most powerful and influential families in Gotham City, the Wayne Family was a target for the rich and malicious Court of Owls. They attempted to have Martha Wayne killed in a car accident and killed Jarvis Pennyworth, Alfred's father and the former butler of the Wayne Family. Eventually, young Bruce Wayne tried to find proof of the Court of Owls but never could.
Years later, the Court of Owls began making significant moves in Gotham City yet again. There were whispers of the Court of Owls' activities, but Batman didn't believe it. It wasn't until Lincoln March (who might actually be Bruce's older brother, Thomas Wayne Jr.) attempted to assassinate Bruce Wayne that he began to consider that the Court was real.
Unfortunately, it was already too late, and the Court captured Batman and trapped him in a labyrinth for days while they made their moves against Gotham City. The Court would later summon the Batman Who Laughs to Prime Earth, but this proved their undoing--as the Batman Who Laughs slaughtered the Court.
7 Creating A Cache Of "Contingency Plans" To Use On Other Justice Leaguers
One of Batman's most infamous missteps was creating his "contingency plans" for the rest of the Justice League. Unfortunately, Ra's al Ghul, who knows Batman's secret identity as Bruce Wayne, was able to find and steal these contingency plans, which included Red Kryptonite for Superman, nanites that cause Martian Manhunter to burst into flames, and a vibrating bullet that causes the Flash to have super-speed seizures. This led to an encounter between Ra's al Ghul and his League of Assassins against the Justice League where all the Leaguers are pacified and almost killed.
Batman is able to distract Ra's long enough for the League to recover, and they manage to survive this assault. This leads to a massive breach of trust between Batman and the Justice League, and the team kicks Batman from the lineup for a while.
6 Falling For The Riddler's Trap During Zero Year
This mistake is less due to poor judgment and more a result of not understanding the Riddler's mind during their first battle, but it is a mistake that nonetheless resulted in Gotham City being plunged into chaos. During Batman's first experience with Edward Nygma, the Riddler uses a jammer to plunge Gotham into a blackout.
Batman tries to shut down the jammer but is distracted by another villain called Doctor Death. Unfortunately, the Riddler expected this and detonated the jammer and took control over the city's power grid. He also detonated the levees and flooded Gotham, and he blocked all entries and exits to the city--effectively holding it hostage. For the next few months, Gotham is completely under the Riddler's control, and it takes Batman some time to recover and strike back.
5 Returning To Gotham City While Thomas Wayne Still Had Alfred Pennyworth
When Thomas Wayne, aka Batman of the Flashpoint timeline, came to Prime Earth, his goal was to stop his son from being Batman, as he believed it to be a terrible burden he didn't want for Bruce. The only way he could think to do this was by breaking Bruce Wayne's spirit (when you're a hammer and all that). He worked with Bane to seize control over Gotham City and had Batman and his allies exiled from Gotham. Thomas held Alfred hostage and threatened to kill him.
Alfred sends a coded message to Bruce letting him know that he escaped from Thomas and Bane, but this was a lie. Alfred didn't want to be used as leverage to keep Gotham under the control of tyrants. Batman and his allies made their move against Bane, and Bane killed Alfred in retaliation. Batman couldn't have known that Bane still had Alfred, but this was a mistake nonetheless.
4 Trying To Stop Damian Wayne From Being Robin
During a war between Batman Incorporated and the League of Assassins, Bruce ordered Damian to stay out of the fight and give up being Robin. He did this for Damian's safety, but he also did it because of a vision he was given of a future where Damian is Batman and does something to cause the President to drop a nuclear bomb on Gotham.
Damian has never been one to sit out of a fight, and Batman should have known this. Damian does join the fight as Robin and ends up fighting an adult clone of himself called the Heretic alone without Batman present. Heretic killed Damian Wayne during this duel.
3 Letting Jason Todd Go After The Joker Alone
During the eponymous Death in the Family story by Jim Starlin and Jim Aparo, Batman and Robin (then Jason Todd) chase Joker across the Middle East and Africa, eventually catching up with him in Ethiopia. Robin chases the Joker and his own birth mother (long story) into a warehouse while Batman goes after a truck full of Joker toxin.
Letting Jason go in alone proves to be a huge mistake, as the Joker captures him, beats him savagely with a crowbar, and then blows up the warehouse, killing Jason Todd.
2 Revealing His Secret Identity To The Joker
One of the more baffling moves ever made by Batman came during one of his earliest encounters with the Joker. After capturing the Joker and sending him to Arkham Asylum, Bruce Wayne went as himself to visit the Joker with a playing card that he had gotten as Batman--essentially letting the Joker know that Bruce Wayne is Batman.
When telling this story to his family, Bruce tried to make it clear that the Joker wasn't capable of processing this connection due to his obsession with the Batman as his own entity. This proved to be a bad gamble, as the Joker has taken advantage of Batman's secret identity both in that story (Death of the Family by Scott Snyder and Greg Capullo) and Joker War by James Tynion IV and Jorge Jimenez.
1 Creating The Brother Eye Artificial Intelligence Satellite
In retaliation for the way the Justice League had taken advantage of him during Identity Crisis by Brad Meltzer and Rags Morales, Batman created new countermeasures for his teammates on the League. He created Brother Eye, an orbiting satellite bearing its own artificial intelligence. Of course, this very quickly went rogue.
Brother Eye went on to create the OMAC army, a vast armada of super-powered drones that hunted down superheroes and converted them into more OMAC units. Furthermore, in the alternate future story called Futures End by Dan Jurgens, Brian Azzarello, Jeff Lemire, Keith Giffen, Andy MacDonald, Patrick Zircher, Allan Goldman, and others, the world is completely destroyed by Brother Eye and its OMAC machines, ultimately creating the world into which Batman Beyond would be born.