The sixth book in the Harry Potter franchise is Harry Potter and the Half-Blood Prince. This book is the penultimate book in the series and it sets up the final battle and journey for the Horcruxes that follows in the seventh book.
It also introduces quite a bit of romance into the series with Ginny and Harry getting together and Ron and Hermione dancing around the truth of their feelings. It's also one of the darker entries into the book and contains one of the biggest deaths of the entire series. We're tracking some of the biggest unresolved plot holes from the book and film.
10 Harry's usage of Sectumsempra
Harry uses Sectumsempra foolishly after discovering it written inside of his potions book by the mysterious Half-Blood Prince. But it's strange that Harry is able to use the spell without having practiced it at all. Even more strange is the fact he doesn't get expelled after experimenting with such dark magic.
We always knew he wouldn't be given he's the main character and his ties to the story, but it's surprising that he wasn't more severely punished for nearly killing Malfoy even if it was an accident and in self-defense.
9 The love potions are a dark inclusion to the story
While love potions are mentioned before the sixth book, this is the first time they really become a big part of the story. Ron even falls under the spell of one after accidentally eating Harry's Christmas gift from Romilda Vane. We also learn more about them from the Pensieve when Dumbledore and Harry visit the memories of Voldemort's past.
It's strange that they're always referred to as love potions because they have a much darker intention. These potions are essentially the magical equivalent of a date-rape drug yet they're advertised openly despite the fact they rob the drinker of free will.
8 Harry doesn't recognize Snape's handwriting
How is it possible that Harry wouldn't recognize Snape's handwriting in his potions book? It should have been fairly obvious to him that Snape was indeed the Half-Blood Prince. There have been numerous times in the books that Harry would have seen Snape's handwriting.
There is even an explicit description of his handwriting at one point in the books. It's surprising that it wouldn't have seemed at all familiar to Harry although it is also possible that Snape's handwriting may have changed over the years.
7 Dumbledore lets Malfoy run wild
It's strange that Malfoy was able to get away with so much stuff during the sixth book. Dumbledore knew what Malfoy was doing and he knew why. There was an underlying plan to allowing Malfoy to continue his many attempts to get the Death Eaters into the school as Dumbledore knew it was going to ultimately wind up in his death.
That said, he also allows Malfoy to seriously injure and almost kill at least two students with a cursed necklace and poisoned mead. You'd think he would have found a way to step in and prevent other students from being harmed.
6 Did the cave potion actually have to be drunk?
During the book's ending when Harry and Dumbeldore actually go on a mission to find one of Voldemort's Horcruxes, they enter a cave and come across a stone basin filled with an emerald potion covering the locket. Dumbledore is forced to drink all of the potions in its entirety before they can retrieve the locket.
The process almost kills him. But did the potion actually have to be drunk? Is it possible that they could have simply dumped it out? Even if it weren't the case, you'd think they would have experimented with some other methods before immediately drinking something Voldemort concocted.
5 The fire at the Burrow
One of the biggest changes in the film version of the sixth book is the scene where the Death Eaters attack the Burrow and set the place on fire. Many fans argue that this scene is strange and out of place with the rest of the movie.
It is also a way to add the Death Eaters into the movie and cut down on the final battle scene. One strange thing about this scene is that no one does anything when the Burrow goes up in flames. You'd think they would be in a rush to put the fire out with water or magical means but no one does.
4 Why didn't they make a replacement locket?
The locket they find in the stone basin winds up being a replacement placed there by Regulus Black. Why on earth did Dumbledore not think to put his own version of a replacement into the basin in the case that Voldemort or someone else ever stumbled on it?
It could have bought them more time if necessary. It doesn't end up being much of a problem in the end but you'd think Dumbledore might have at least considered it. Even Hermione creates a locket replication later on when stealing from Umbridge.
3 Why don't the Death Eaters kidnap Harry?
During the ending of the film, Harry is incapacitated and laying on the ground. It would have been an opportune time for Bellatrix or some other Death Eater to simply kidnap him and take him to Voldemort.
It seems strange that they would have wasted such a valuable moment and left him laying on the ground. We know that Snape barked that Harry was meant for the Dark Lord but that doesn't mean they couldn't have taken him to him.
2 Why would Merope name her son after her father?
Why on earth would Merope name her son, Tom Marvolo Riddle, after her father? Merope's father, the Gaunts, were extremely abusive to her. She lived her life in misery and squalor.
She was outspoken about her hatred for them and eventually concocted a love potion to make a handsome Muggle she had become obsessed with fall in love with her. But it seems strange she would commemorate her family by adding that middle name to her son.
1 What was in the Malfoy's secret chamber?
The Malfoy's secret chamber is mentioned a couple of times in the novel. It is said they keep a secret room full of ancient and dark artifacts and yet when the Ministry raids their home they find nothing.
They go through several raids where they try to find something off in the Malfoy home but can't manage to discover their vast secrets. It could be that they find it later one as Lucius does wind up getting fired but it does seem strange we never find out the exact details.