A good character has to be written with a variety of faults. This makes them interesting and, ultimately, more realistic. In the Harry Potter series, JK Rowling did a great job of creating characters that people could relate to, and see themselves being friends with, and one of these characters is Hermione Granger.
Hermione does a lot of growing through the series. She starts off struggling to make friends due to her tendency to behave a little bit like a know-it-all, but she doesn't eventually make connections. However, there are times throughout the series that Hermione's know-it-all nature takes over and leads her to be a poor friend. In both the books and the movies, Hermione's quotes prove that some of her worst characteristics can sometimes result in her letting her friends down.
She's Overly Critical
"Are You Sure That's A Real Spell? Well, It's Not Very Good, Is It?"
Harry and Ron first met Hermione on the Hogwarts Express and she didn't exactly rub them the right way. She criticized Ron's wand work, showed off some of her own (illegal) magic, and embarrassed Ron by pointing out his dirty face.
While the trio was not officially friends yet, Hermione's first experience with the boys was not far off from what they would grow used to in later years. She could be extremely critical of them. This quality can make Hermione the funniest character in Harry Potter, but it can be a point of frustration for her friends.
She's Judgmental
"Honestly, Don't You Two Read?"
All fans know that Hermione spends most of her time with her nose in a book. While she has been criticized for experiencing the world this way, she still often takes the angle that her knowledge from books, as opposed to real-world knowledge, is superior.
Ron and Harry are some of the smartest characters in Harry Potter, but they never felt the need to pour over books like Hermione. However, she still judges their decision to not read day in and out and ridicules them for not having the knowledge that she would have just shared with them anyway.
She Is Far From Comforting
"No, Harry. Even In The Wizarding World, Hearing Voices Isn't A Good Sign."
A logical mind can be good to have around, but logic can have its downsides. Friends don't always need cold logic to solve their problems, but could instead use a little empathy and tact.
When Harry shares that he has been hearing voices, Hermione encourages him to keep it to himself. If she had not scared him into staying silent, he might have taken his fears to Dumbledore, and the whole "Champer of Secrets" situation could have been avoided.
She Is Impatient
"Yes! I've Already Agreed It Was Fishy Harry!"
Hermione doesn't enjoy entertaining other people's ideas. Once she has decided that someone is wrong, she will quickly become irritable if they continue to push the matter. For this reason, it seemed as if some of her friends at Hogwarts would have been better off without her.
This is clear many times throughout the series when she disagrees with one of Harry's theories. When Harry was sure that Malfoy was a Death Eater, Hermione became exasperated and snapped at Harry, stating that she agreed that Malfoy's behavior was suspicious, but refusing to entertain Harry's theory (which ended up being correct).
She Disregards The Opinions Of Others
“But He Didn’t Get The job, Did He? So He Never Got The Chance To Find A Founder’s Object There And Hide It In The School!”
In Deathly Hallows, Harry was convinced that a Horcrux was hidden at Hogwarts. The trio had been trying to figure out where to look to no avail, so when Harry suggested the school as an option, he should have been taken seriously.
However, Hermione had decided that this was impossible. She felt that Voldemort had never had time to hide a Horcrux there, so searching the castle was out of the question. It didn't matter that Harry's opinions had often been proven to be accurate, her opinion was all that mattered. The decision to not search Hogwarts sooner was one of the most frustrating decisions that Hermione made, but to be fair, things might not have worked out as well if they had.
She Puts Her Anger In The Wrong Places
"Don't You Tell Me What To Do, Harry Potter! Don't You Dare!"
When Ron left the group in Deathly Hallows Hermione was heartbroken. Harry had to deal with a lot of guilt, knowing that his fight with Ron had put him over the edge. He provided her with comfort as best he could, such as in the famous dancing scene from the movie. When Harry found Ron in the forest, he was excited to bring him back to the tent and show Hermione who had returned.
However, Hermione took her anger at Ron out on Harry as well. Harry tried to stand up for Ron, but Hermione would not have it. Despite how he had cared for her, she didn't treat him very well at that moment.
She Doesn't Trust Others' Intuition
"You Just Need To Practice. It's All A Matter Of Confidence Harry"
When Harry and Hermione were attacked by Nagini at Godric's Hollow, Harry's wand ended up getting broken by one of Hermione's rebounding spells. She felt terrible about it and apologized repeatedly. Harry knew it wasn't entirely her fault and did his best not to take his anger out on her.
What frustrated him, however, was the way that Hermione refused to believe that the wand that Ron had stolen from a snatcher would not work for him properly. Since he only had his intuition and no hard facts, Hermione disregarded him and told him to just practice.
She Won't Admit When She Could Be Wrong
"But That's - I'm Sorry, But That's Completely Ridiculous! How Can I Possibly Prove It Doesn't Exist?"
When Hermione is told to prove that the Hallows don't exist, she becomes frustrated. She states that with that logic, anyone could believe in anything since most things cannot be proven to not exist. Xenophilius tells her that this is precisely true, calling her closed-minded.
This enrages Hermione, but Harry realizes that Xenophilius was right. Hermione was incapable of thinking outside the box of reason. If something didn't line up with her idea of truth, she could not bring herself to say that she might be wrong about it.
She Is Overly Stubborn
"But The Deathly Hallows Can't Exist Ron!"
After the trio gets all the information they can about the Deathly Hallows, they all argue about whether or not they are real. They all agree that Harry's cloak is a lot like the tale, but Hermione is still determined that the Resurrection Stone and Elder Wand were impossible.
No matter the evidence the others gave, or how much they tried to convince her to open her mind to the possibility, she would not budge. Of course, she was wrong, but her stubbornness led her to treat her friends rather poorly over the issue.
She Can Be Manipulative To Get Her Way
“But I Only Said That To Try And Persuade You To Come To The Lovegood's! I Didn’t Really Believe It!”
Hermione had been sure that the Deathly Hallows were important but Harry was not convinced, feeling that Dumbledore would have told him about them if they were. Hermione used his own words against him, telling him that Dumbledore likely didn't tell him about the Hallows since they are something that he needed to work out for himself.
After talking to Xenophilius, Harry and Hermione switched places. Harry was convinced of their importance, while Hermione found the whole story laughable. Harry pointed out what she had said before, and in frustration, she admitted to manipulating him to get what she had wanted. This was especially frustrating to Harry, considering all the leads Hermione had disregarded, only for her to go to such length to explore her own ideas.