1The Dursleys were mean because they were under the effect of a Horcrux
2The Deathly Hallows refers to Voldemort, Snape, and Harry, and Dumbledore's Death.
3Dumbledore is a time-traveling version of Ron Weasley
4There might have been a war between the muggles and wizards, with the muggles being the winner
5There are ghosts of former Hogwarts students that died there
6Draco Malfoy is a werewolf
7Ginny may have given Harry a love potion
8The Defense Against the Dark Arts position was cursed by Voldemort
9Harry is the Heir of Gryffindor
10Moaning Myrtle was an urban legend
11Neville wasn't bad at magic, but he was using the wrong wand
12Harry never left the cupboard under the stairs
13Neville Longbottom is the real hero
14The Sorting Hat sorts students based on what traits they value, not possess
15Harry is now immortal
16Crookshanks is actually the Potters' cat.
17Rita Skeeter is J.K. Rowling
18Dramione (Harry Potter)
19Luna is Autistic
20Dumbledore is gay
21George Weasley is Willy Wonka
22Seamus Finnigan and Dean Thomas are gay for each other
23Dudley saw his bratty and bullying memories with the dementor and it terrified him
24Muggles hated The Fat Friar and Sir Nicholas because they used magic so the two were executed
25Minerva McGonagall is a death eater
26Dumbledore had a horcrux
27Umbridge was muggleborn, but lied about it to avoid arrest
28Crabbe and Goyle have troll ancestors
29Hermione wrote the entire series and then let J.K. Rowling (who she met on a train and felt bad for) have all the credit so that it could be sold as fiction even though every word of it is true.
30Voldemort does know love, he just doesn't want it to ruin his dark personality.
31Crabbe and Goyle are cousins
32Harry, Ron and Hermione asked to be in Gryffindor