The movies fudged the discovery of the horcruxes
I've been doing a tandem rewatch/reread with the release of the full cast audiobooks. I just finished listening to HBP and am now watching the movie.
Something I missed in previous watches is just how royally they messed up the research and discovery of the Horcruxes that Albus and Harry do.
In the book, Dumbledore knows ahead of time that Voldemort was looking to create Horcruxes. In the altered memory he had from Slughorn, Tom Riddle says the term "Horcrux" but it stops short of them explaining what Horcruxes are. Obviously, Dumbledore has been around long enough to know what they are, especially if Slughorn had that knowledge. The point of getting the unaltered memory wasn't to learn what a Horcrux was, but to learn HOW MANY Horcruxes Voldemort had intended to create. Without that information, they wouldn't have known when they had destroyed them all and could then attack Voldemorts physical form.
In the movie, they make a point to blur out the term "Horcrux" from the altered memory. Instead, they save it for the complete memory Harry gets from Slughorn. They leave out the discussion of how many Horcruxes Riddle would want to create. Thus, they make it seem like Slughorn has information on this dark form of magic that even Dumbledore doesn't have.
I suppose in the overarching plot, it doesn't make a difference, but it does create doubt in Dumbledore's vast knowledge, and feels like the movie producers kind of missed the plot on that whole storyline.