1. Sirius Black, Remus Lupin, James Potter, and Peter Pettigrew were actually the ones who created the Marauder's Map when they were students at Hogwarts.
2. Albus and Aberforth Dumbledore had an intense sibling rivalry after their sister, Ariana, accidentally killed their mother. They competed over who cared the most about their sister. They dueled against each other, and when Ariana tried to stop them, Albus accidentally killed her.
3. Harry, Ron, Hermione, and the Weasley family visited Arthur at St. Mungo's Hospital after he was attacked in the Department of Mysteries. They noticed Neville in the same ward, visiting his parents who were very much alive but driven "insane" by Bellatrix's Cruciatus Curse.
4. When Voldemort died in Deathly Hallows, he didn't disintegrate into a million little pieces — he died a mortal human being.
5. The two-way mirror Harry used in Deathly Hallows was actually given to him by Sirius Black in Order of the Phoenix, just in case he needed to ask him for help.
6. Because Hermione always wanted to be the best in her class, her boggart in Prisoner of Azkaban was Professor McGonagall announcing she failed all of her tests/courses.
7. Not only did Professor Umbridge send the Dementors to torture Harry and Dudley in Little Whinging, but she also gave Fred, George, and Harry a lifetime ban from Quidditch.
8. After working under Barty Crouch Sr., Percy returned to the Weasley family in Deathly Hallows and told a funny joke to Fred just before he died.
9. In Chamber of Secrets, Peeves (a mischievous poltergeist at Hogwarts) broke the vanishing cabinet to distract Filch from yelling at Harry. This ended up being the same cabinet Malfoy repaired in Half-Blood Prince in order to kill Dumbledore.
10. Bellatrix Lestrange was actually Nymphadora Tonks's aunt. Nymphadora's mother, Andromeda, was Bellatrix's younger sister.
11. Harry's first official date was with Cho in Order of the Phoenix. They went to Hogsmeade and drank tea on Valentine's Day.
12. Nearly Headless Nick hosted a Deathday Party for himself on Halloween in Chamber of Secrets to celebrate his 500th anniversary of death.
13. The Bloody Baron (Slytherin's ghost) was deeply in love with Helena Ravenclaw (Ravenclaw's ghost). When she ran away to Albania to get away from her mother, Rowena Ravenclaw, the Bloody Baron asked her to return with him. When Helena refused, he killed her and then himself.
14. Hogwarts students and professors held a funeral for Dumbledore when he died and gave him a proper farewell at the Black Lake.
15. During the Battle of Hogwarts, Harry invisibly punched Malfoy in the face.
16. The way to enter the kitchens at Hogwarts was through a fruit portrait in the entrance hall. Once you tickled the pear in the portrait, the door would open.
17. And Harry gave the money he won from the Triwizard Tournament to Fred and George, and they used it to open up their joke shop: Weasleys' Wizard Wheezes.
Note: Submissions have been edited for length and clarity.