When Dumbledore says Hogwarts has a "hidden swimming pool", he's not kidding. Harry visits the "hidden swimming pool" (or giant bathtub, either one) in the prefects' bathroom in the fourth book.
Snape shows Harry exactly what he needs to do. Die, of course!
Bellatrix tells Voldemort there are "pieces of [him] missing", and Voldemort defensively asks if she's talking about his Horcruxes. Considering how Voldemort survives, she is talking about one of them.
In Ron's first year he was told he doesn't get the girl by future!Draco. So he's a bit of a butthead — he's protecting himself by denying any feelings for Hermione until Granger Danger strikes and he can't deny anymore.
This fits with how in AVPSY, where they are together, he's no longer mean to her at all (not counting his "affair" with Lavender.)
There are a few clues that Scabbers isn't Peter Pettigrew. First Ron mentions that his parents found Scabbers the same night that Harry's parents died. In the books, it was a few days before Sirius caught up to him and was framed. Second, Sirius used the Invisibility Cloak to escape Azkaban (somehow) instead of escaping as a dog, suggesting that the Marauders didn't become Animagi.
According to J.K. Rowling, The Chamber of Secrets was originally going to be called "Harry Potter and the Half-Blood Prince". This eventually became the title of the sixth book, which also contained some of the plot points that originally was a part of the second book. This play contains elements from both of these books. It also implies that the Harry Potter books were originally written by Hermione as a presumably truthful account of Musical Harry's adventures, meaning that it too would be a mix of the second and sixth books. Her essays were then heavily edited by Gilderoy Lockhart, who dramatically changed the order of which the events unfolded, creating our universe's Potter books while splitting up the plot of AVPSY into two separate stories.
If one interprets Quirrell's love saving Voldemort as Quirrell literally being a horcrux. That means that it would be literally impossible for Voldemort to die unless Quirrell does. Sounds kinda awesome, right? But if one mistakenly believed that Voldemort was still evil and needed to kill him, they would first need to murder the first and only person that Voldemort ever felt a shred of compassion or friendship towards