Albus Dumbledore has a scar above his left knee that is a perfect map of the London Underground.
2. J.K. Rowling stated recently that the reason that Vernon Dursley hated the Potters so much was not because they were wizards, but because on a double date with James and Lily, James made him feel stupid and embarrassed. Vernon doesn’t like to be reminded of it, and therefore doesn’t like Harry because of his resemblance to James.
3. Before Daniel Radcliffe played the lead role in the Woman in Black (2012), Adrian Rawlins (Radcliffe's father in the Harry Potter series) played the same role in the original 1989 film.
4. The most commonly used sentence in the Harry Potter books was: “Nothing happened.”
5. To protect the final Harry Potter book from leaking to the public, a 10-million pounds security operation featuring satellite tracking systems and armed security with K-9 units was used. A Scholastic lawyer also physically sat on a copy during a flight to prevent any potential loss.
In the Harry Potter books, Parvati Patil of Gryffindor has an identical twin Padma Patil of Ravenclaw, to illustrate the unpredictability of the Sorting Hat. In the movies, however, Parvati (Shefali Chowdhury) and Padma (Afshan Azad) are not only both in Gryffindor but are played, surprisingly, by unrelated actresses.
7. In the movie adaption of Goblet of Fire, the audience is given the impression that Beauxbatons is an all-girls magical academy whereas, Durmstrang is an all-boys one. In the books, however, both schools are co-ed, and in fact, in the books, the Patil twins leave Harry and Ron to spend time with boys from Beauxbatons.
8. In the novel, Hermione uses the Sleekeazy's Hair Potion, a hair care potion, to straighten her hair for Yule Ball. J.K. Rowling would later reveal on Pottermore, as part of the Potter family background, that this potion was invented by Fleamont Potter, paternal grandfather of Harry Potter.
9. In the fourth movie, the Weasleys have really high seats at the Quidditch World Cup. The Malfoys make fun of them for being so high up in the "nosebleed section." In the book however the Weasleys have just as good as or even better seats than the Malfoys. The tickets were given to Arthur Weasley by Ludo Bagman at the Ministry of Magic, whose character is cut out of this movie due to time. Ludo's character is also a degenerate gambler, and throughout the book tries to help Harry cheat in the Triwizard tournament; Harry refuses his help.
10. While filming Order of the Phoenix, Alan Rickman banned Matthew Lewis (Neville Longbottom) and Rupert Grint (Ron Weasley) from being within 5 meters of his new BMW, because during the making of Harry Potter and the Goblet of Fire (2005), they spilled milkshake in his car.
11Dobby and Winky
Some scenes of the elves Dobby and Winky were cut from the Goblet of Fire due to time constraints. However, if you watch carefully in the first campsite scene, right after Ginny points to something and says "Look!" you can see two House Elves riding on llamas. They go by very fast, so they're hard to see.
12. J.K. Rowling provided over 70 names for the Black family tree tapestry, complete with details of relations between each and every member, whose were to be scorched and so on.
13. Several members of the Black family are named after astronomical objects. You can see some of the names in the scene when Sirius and Harry are in the family tree room. Sirius, Regulus, Andromeda, Arcturus, Bellatrix, and Cygnus are the ones that can be seen.
14. According to the original script, the character of Kreacher was not intended to be in Order of the Pheonix at all. But after J.K. Rowling read the script, she more or less insisted on him needing to be there to avoid some serious problems with future installments of the series. Though Kreacher has no noticeable impact on the plot or story as it's presented in the film, a couple of scenes with him were added at the last minute based on this request.
15. During the breakfast scene of the Great Hall in Order of Phoenix, boxes of cereal can be seen with the names Cheeri-Owls and Pixie-Puffs, with color schemes similar to those of Cheerios and Sugar Puffs boxes, respectively.
In Order of the Phoenix, Mr. Weasley dials 62442 in the “old red telephone box” to get to the Ministry of Magic for Harry’s trial. When typed into a modern cell phone on predictive text, this number comes up as “MAGIC.”
17. In Harry Potter and the Order of Phoenix, when they discover the Room of Requirement, Ron asks if it could become a bathroom if the user really needed it. This is a reference to the novel 'Harry Potter and the Goblet of Fire'. In it, the Room of Requirement is mentioned for the first time, having become a lavatory for Dumbledore.
18. Padfoot, Sirius Black's canine form, animagus, was played by a Scottish Deerhound named Cleod.
19. J.K. Rowling had been a schoolteacher before writing the Potter books, and Order of the Phoenix features some references to the British educational system. In the late 1980s and early 1990s, students at age 16 were given Ordinary Level (O-Level) exams in all their major subjects, for which they can receive their General Certificate of Education. This corresponds to the Ordinary Wizarding Level (O.W.L.) exams given at Hogwarts. Students who planned to go on to university stayed on two more years to take their Advanced (A-Level) Exams, which roughly correspond to N.E.W.T. (Nastily Exhausting Wizarding Test) Exams.
20. Over 30,000 individually placed tiles were used to create the Ministry of Magic sets in Harry Potter and the Order of the Phoenix. As real ceramic tiles would have been far too expensive to produce, they were instead made out of heavily-painted cardboard.
Author J.K. Rowling revealed that the character of Dolores Umbridge in the Harry Potter series was based on a real person whom she "disliked intensely on sight", and whom "returned [her] antipathy with interest", even though she honestly could not explain why they hated each other so much.
22. When Dumbledore's Army first meets at the Hog's Head Inn in Order of the Phoenix, there is a bartender accompanied by a goat, played by Jim McManus, and credited as "Barman." This character is later identified as Aberforth Dumbledore, the Professor's younger brother. This character returns in Harry Potter and the Deathly Hallows: Part 2 (2011), and is played by Ciarán Hinds.
23. In the books, the entire plot of Goblet of Fire is foreshadowed when Harry and Ron are doing their divination homework. While they're doing their divination homework, Harry and Ron mention: a "danger of burns" (1st task), losing a "treasured possession" (2nd Task), being "stabbed in the back by someone you thought was a friend" (either Barty Crouch Jr. or Ron being an ass after Harry was chosen as champion) and coming "off worse in a fight" (the fact Voldemort comes back and can now touch Harry).
24. Timothy Hunter is a DC Comics character created by Neil Gaiman seven years before Harry Potter and the Sorcerer's Stone. He wears glasses, lost his mother when he was young, discovered he was destined to be a powerful wizard and has a pet owl.
25. The Harry Potter series has sold more books than Lord of the Rings, Narnia, Hunger Games and Twilight combined.