The recent release of Fantastic Beasts: The Secrets of Dumbledore has further demonstrated to audiences how even the most influential characters can fall prey to bad decisions. Albus Dumbledore is considered one of, if not the most powerful wizards in the wizarding world, but many of the books and films in the series revolve around his character flaws. He has advised other characters not to follow in his footsteps and choose what is right, not easy, but this is often difficult to do.
Students in Harry Potter can always be expected to make poor decisions, but professors are considered to be the voice of reason. However, no one is perfect, and every Hogwarts professor had a moment of failure in their job. Some professors, like Dolores Umbridge, had poor intentions from the start. Others, like Professor McGonagall, always wanted what was best for their students but were doomed by human nature. No matter their purposes, several Hogwarts professors made poor decisions at some point during their post, ultimately proving that even teachers aren't perfect.
Sybill Trelawny - Exaggerating Her Abilities
Divination is a subject that very few witches and wizards will ever have a natural gift for. While Professor Trelawney knew this, she still chose to pretend that she had far greater ability than she did, which wound up getting her into a lot of trouble.
When Umbridge was auditing classes, Trelawney could have been safe and taught her students by the book. Instead, she greatly exaggerated her abilities and set up expectations that she could not possibly fulfill. She temporarily lost her job due to her dishonesty, which may not have happened if she hadn't tried to show off.
Gilderoy Lockhart - Becoming A Teacher To Begin With
Gilderoy Lockhart had a pretty good life set up for him. He had conned his way into fame and had the entire wizarding community believing that he was a brave and powerful wizard. If he had continued along this path, he likely would have lived a long and happy life in the spotlight.
Lockhart's biggest mistake as a teacher was becoming a teacher. He took on the post of Defense Against the Dark Arts professor, a position that was all but guaranteed to reveal him for who he truly was. This horrible decision put him in St. Mungos, utterly unaware of who he was.
Horace Slughorn - Letting His Sycophantic Tendencies Cloud His Judgment
As a classic Slytherin, Horace Slughorn was greatly seduced by power and influence. However, he preferred to sit back in luxury instead of putting in the hard work of politics, while his powerful companions sent him gifts and granted him power by proxy.
Slughorn achieved this lifestyle by developing personal relationships with students in which he saw potential. While every teacher has students they are fond of, Slughorn let his thirst for glory cloud his judgment with his students. This ultimately made it easy for the likes of Riddle and Harry to manipulate him. If he had kept a professional distance, that would not have been the case.
Armando Dippet - Favoring Tom Riddle
Professor Dippet was the Headmaster before Dumbledore. He was described to have been completely charmed by Riddle and favored him above other students. This means that when Riddle came to him accusing Hagrid of killing Moaning Myrtle when she was still a student, he believed him wholeheartedly.
Headmasters should remain as neutral as possible so that they can make fair decisions in these situations. In the end, Dippet's poor decision to favor one student over another led him to expel an innocent boy with no proof that he had committed the crime.
Rubeus Hagrid - Confiding In Students
Harry Potter fans might argue that Hagrid has the best heart of the series, but he is far from perfect when it comes to professionalism. While his friendship with Harry was valuable, it often led him to confide in him far more than a Hogwarts professor ever should.
While he was a professor, Hagrid revealed to Harry that he had overheard Dumbledore and Snape in an argument. He quickly realized that this was a mistake, but it could not be taken back. Instead of ending the conversation there, he went on to explain everything he had heard. While Hagrid is a favorite character in the Harry Potter series, and it's hard to blame him, it is clear that revealing this information to a student was unprofessional.
Dolores Umbridge - Overestimating Her Superiority
Dolores Umbridge is the quintessential example of lawful evil. She plays by the book so she can always justify her actions even if several people end up hurt. However, Umbridge's greatest mistake is believing herself superior because of her blood status and position in the Ministry.
Professor Umbridge used her extensive rules and regulations to quickly climb to the top of the Hogwarts chain of command. However, her social power made her forget that she had no remarkable skill and was likely one of the worst duelists in the Harry Potter series. Umbridge believed that her superiority gave her the right to be abusive to students and other teachers, but she quickly learned that her status wouldn't protect her from the repercussions.
Minera McGonagall - Ignoring Harry's Warnings
Professor McGonagall may have come across as stern, and she was, but she would also go to any lengths to help her students succeed. However, when Harry would come to her for help, she sometimes made the wrong choice about how to handle it.
McGonagall first dismissed Harry when he told her that someone was going to try to steal the Philosopher's Stone, and again when he confided that he suspected Malfoy to be behind the attack on Katie Bell. While McGonagall was correct that both situations were under control, she should have realized that Harry would take things into his own hands if he wasn't listened to. By dismissing Harry, McGonagall ensured that the Boy Wizard would put himself in danger.
Remus Lupin - Concealing Important Information From Dumbledore & Endagering Students
While Remus Lupin was the best DADA professor that Harry ever had, he was far from a perfect character and repeatedly ignored his own advice in the Harry Potter series. In Prisoner of Azkaban, Lupin knew that Sirius was evading capture because he was an Animagus, but he was too afraid to tell Dumbledore because he feared how that would reflect on him.
If Lupin had made the right decision to tell his boss that his childhood friends could turn into animals and that Peter Pettigrew had been seen on the map, Dumbledore likely would have been able to put the pieces together and liberate Sirius before everything went downhill. But unfortunately, Lupin chose secrecy instead, and Sirius suffered as a result. What's more, Lupin put students' lives — particularly Harry's — in grave danger by not spilling the beans.
Severus Snape - Putting Personal Feelings Ahead Of Teaching
When Harry was using the Half-Blood Prince's book, he told Hermione that he had learned far more from the Prince than he had ever learned from Snape. This was ironic since they were, in fact, the same person.
Snape was a potions genius, and if he had truly applied himself, he could have been a fantastic teacher. However, only some student's benefitted from his lessons. Harry could have learned so much more in Snape's class, but the professor's hatred for Harry's father prevented him from making a reasonable effort. Snape also had it out for all Gryffindor students, ensuring that none of them would ever find value in his class.
Albus Dumbledore - Favoring Gryffindor Students
As the Headmaster, Dumbledore should have approached the different houses of Hogwarts with neutrality. However, this was never the case. As a Gryffindor himself, there were several instances that he proved his old house was the one that he preferred.
The clearest example of this was the awarding of the House Cup at the end of each school year. In Philosopher's Stone, Slytherin had won the Cup by a considerable margin. However, Dumbledore made sure to give just enough extra points to the Gryffindors to give them victory. While audiences were cheering for Harry and co. at the time, the preferential treatment was really not fair in hindsight.