History Trivia / HarryPotterAndTheHalfBloodPrince

* CaliforniaDoubling: The only time in the series this happened; the exteriors for the cave (which is Cornwall in the book) were filmed in the West of Ireland.

* IKnewIt: Before the release of the book, oddsmakers had [[spoiler:Dumbledore]] as the character most likely to be killed and they were right.

* WhatCouldHaveBeen: See [[WhatCouldHaveBeen/HarryPotterAndTheHalfBloodPrince page]].

* ReleaseDateChange: The film was originally scheduled for a November 21, 2008 release, but it was later delayed until July 15, 2009. The reason for this is that the [[UsefulNotes/TVStrikes 2007-08 writers' strike]] had left Creator/WarnerBros without a tentpole release for '09, and Warners had already had a tentpole release in '08 with ''Film/TheDarkKnight''. As a result of this, ''Half-Blood Prince'' didn't come out until over a year after it had actually been filmed, by which time they were already well into the filming for ''[[Film/HarryPotterAndTheDeathlyHallows Deathly Hallows]]''. Meanwhile, Creator/SummitEntertainment decided to move a little film of theirs called ''{{Literature/Twilight}}'' into the vacant November 21, 2008 slot.

* WhatCouldHaveBeen: See [[WhatCouldHaveBeen/HarryPotter subpage]].

* AuthorsSavingThrow:
** This book contains two subplots where Love Potions are used in situations that are ''definitely not'' DoubleStandardRapeFemaleOnMale, in a possible response to criticism that said Potions are basically G-rated date rape drugs marketed to Potterverse witches (for added contrast, Fred and George do exactly that in this book and it's just as creepy as it sounds). In the present day, Romilda Vane is seen as a disturbed stalker after she tries to send Harry a box of chocolates that were spiked with Love Potion; and in the backstory, [[spoiler:Merope Gaunt, Voldemort's mother, kept Tom Riddle under various enchantments, including Love Potion, so he wouldn't leave her]].
** The Horcruxes are established in this book and it's retconned that Riddle's Diary in ''The Chamber of Secrets'' was a Horcrux. Rowling is aware that considering how precious Horcruxes are established to be it's illogical that Voldemort would make one that would be much more obvious and open to discovery. Dumbledore gives a throwaway line in the book that Voldemort was such an egomaniac that he created a Horcrux that would be discoverable just because he could proclaim himself to be the "Heir of Slytherin".
** The scene where Hermione attacks Ron with the birds is not very popular, due to DoubleStandardAbuseFemaleOnMale (had Ron attacked Hermione like that, it'd be a MoralEventHorizon). In the film, Ron is not hurt when the birds attack.
** A big complaint about Harry falling for Ginny is that her CharacterDevelopment happens mainly offscreen. The film shows Ginny to be a supportive presence to Harry, featuring her a lot more in scenes and giving them a chance to develop a relationship on screen (especially after barely interacting in previous films). The film also excises Harry breaking up with her at the end, an act that some feel was unnecessary drama on top of everything else.
** A major complaint for the book series is that the [[AmbitionIsEvil Slytherins]] are uniformly underhanded cheaters, opportunistic bullies, and snobby [[AlphaBitch Alpha Bitches]], while the Gryffindors are all brave, virtuous champions. Many readers and even casual fans have joked why Hogwarts has a house specifically for the evil students. The book attempted to reverse this by introducing Horace Slughorn, who while still ambitious and opportunistic is also friendly and jovial. The Gryffindors got Cormac [=MacLaggen=] and Romilda Vane, who embody their house's virtue of bravery but are also a selfish JerkJock and creepy StalkerWithACrush respectively, with Vane also being a bit of an AlphaBitch in the book (inviting Harry to come sit with her friends by saying he won't have to sit with ''them'', meaning Neville and Luna).
** The film leaves out Harry trying various spells from the book on other people, showing only the [[spoiler: Sectumsempra scene]] which in-universe is meant to be horrifying.
** A minor one, but some fans felt that Creator/HelenaBonhamCarter was far [[EvilIsHammy too hammy]] playing Bellatrix in the previous film. Here she is much more subtly menacing and restrained, while still being evil and crazy.
** Likewise, the [[AdaptationalJerkass infamously mean-spirited]] aspects of Creator/MichaelGambon's Dumbledore is notably toned down from previous films, with Gambon delivering a more subdued, gentle performance closer to [[TruerToTheText the books]] and [[Creator/RichardHarris his predecessor]].
** The relationship arcs in the novel were polarizing among readers upon release, feeling the characters -- Ron and Hermione in particular -- were bordering on {{Wangst}} with their drama. The film attempts to rectify this with Steve Kloves rewriting virtually all of those segments with brand-new dialogue, with a heavier emphasis of comedy, making each character's ill-fated relationships go down easier by virtue of making them funny with an occasional genuine beat of heartbreaking.