Nice Job Breaking It Hero / Harry Potter

• In the first book Harry rushes to save the Philosopher's Stone from Voldemort. Turns out the Stone was hidden so that only a "pure of heart" (basically) could retrieve it, which Harry does, and the Stone nearly falls into Voldemort's hands.
• In the second both Harry and Ginny write in Tom Riddle's diary, unknowingly giving him valuable intelligence and almost allowing him to become corporeal.
• In the third, Harry spares Wormtail by proxy, only to have him escape and help ensure Voldemort's return, but hey, at least Buckbeak and Sirius live!
• Goblet of Fire: Harry comes up with the idea of both he and Cedric grabbing the secretly-transportational trophy simultaneously; once they arrive at its destination, Cedric is promptly murdered by Pettigrew under Voldemort's orders. Whoops. Of course, if just Cedric had grabbed it, things might have turned out even worse since there wouldn't have been anyone to alert the wizarding world (or at least Dumbledore) of Voldemort's return.
  • Then again, neither had any idea what was going to happen. BOTH of them (and everyone else to boot) fully expected teleportation to the winner's podium, not to a lethal confrontation with the resurrected Dark Lord.
• Harry Potter and the Order of the Phoenix:
  • Worst of all in Order of the Phoenix, where Harry's desperate attempt to rescue Sirius winds up with his friends and drawing Sirius to his death, instead. The upside — Voldemort being exposed as alive — pales in comparison to the loss; ironically, since the entire book was about Harry's frustration with not being believed and attempts to make people believe him, only for Harry to not give a damn anymore when he was shown to be right.
  • And, to a lesser extent, in the same book, when they manage to break every damn prophecy and all the Time Turners in the Ministry of Magic.
  • At Christmas in OoTP, Harry is sent a present from Sirius that he doesn't open until after Sirius' death. It's one of a pair of communication mirrors -and Sirius had the other one. Harry could have talked to Sirius to check on him ages ago!
• Harry Potter and the Half-Blood Prince:
  • At one point in Half Blood Prince, we see inside the memories of a Ministry official who was responsible for the arrest of a father and son who abused the daughter of the family, enabling her to go after the Muggle love of her life. All very well, right? Well, there's just one teensy-weensy problem — said daughter happens to go on to become the mother of the most evil and maniacal wizard this century after using a Love Potion to turn her crush into her Sex Slave.
  • In the same book we learn R.A.B. stole a Plot Coupon and replaced it with a fake and a taunting note, which forces the main characters to find it again later because R.A.B. didn't manage to destroy it. Especially annoying because of the circumstances of pointlessly gaining the fake.
• Harry Potter and the Deathly Hallows:
  • Subverted in the Deathly Hallows. Harry realizes he must let Voldemort kill him. As this was all part of the plan, and Harry suffered no more than a bruise, it all worked out.
  • In a more subtle example, the majority of the population tends to call the Big Bad by impersonal aliases, like You-Know-Who or The Dark Lord, while Dumbledore calls him by the name and encourages others to follow suit. This backfires in the last book, when Voldemort charms his own name, so that whoever utters it will be stripped of all wards (except the Fidelius Charm) and exposed to his minions. Which leads us to Harry blurting out Voldemort's name despite just being told not to resulting in each and every magical defence around their tent all crashing down at the exact same time. You could construct a very decent argument here that this makes Harry directly responsible for Dobby's death.
  • Also in the last book, not that he was much of a hero in the first place, Mundungus Fletcher's cowardly and stupid act of chickening out by disapparating inadvertently got Mad Eye Moody killed, arguably Mundungus' worst mistake. He then followed up with another blunder—stealing Salazar Slytherin's locket as if it was something for the market and end up being bribed into giving it to Dolores Umbridge, who he was possibly unaware had become loyal to Voldemort's new regime. After those huge blunders, Harry, Ron, Hermione and everyone else would refuse to work with him again.
  • To a lesser extent, Harry's decision to spare Dolohov and Rowle and erase their memories instead of killing them when they had the chance. Dolohov would end up killing Remus during the Battle of Hogwarts.
  • Back in his school years Hagrid releases his pet giant spider into the Forbidden forest to preserve it from being killed and even finds it a mate. Fast forward and, what a shock, they spawn a swarm of vicious monsters, whom the Death Eaters later use against the school in the Final Battle.