A trait of J.K. Rowling is to make up names that are pertinent or descriptive of their object/person (eg. Diagon Alley, Voldemort, etc). This is the same for Knockturn Alley: It is a place you wouldn't want to visit 'nocturnally'.

In Dumbledore's office at the end of the movie, Malfoy says, "Let us hope that Mr. Potter will always be around to save the day." Harry says, "Don't worry. I will be." These lines were not in the script but made up when they were filming.

The first scenes in the movie are some of the last scenes filmed. Viewers kind of tell by the way the actors' voices and looks changed. For example, Matthew Lewis (who played Neville Longbottom) said that he had to stuff cotton balls in his cheeks in the "Mandrakes in the Greenhouse" scene, since he had lost so much weight since the beginning of filming.

When Harry, Ron and Fang begin to walk through the Dark Forest there is a close-up of Fang walking towards the camera. At the start of the next shot, at the bottom right of the screen the Ford Anglia is visible. Although the scene of them seeing the car before it rescues them is deleted, this one shot remains of the car 'watching' them.

In the scene where Harry and Malfoy are flying on their brooms, there's a shot where Colin takes a picture. Listen to the music right after that. The music is the same music used in Star Wars: Episode II - Attack of the Clones during the chase scene on Coruscant (John Williams did the music for this film and all the Star Wars films).

There is an extra scene after the credits. You see the after affect of Lockharts backfire of the 'Obliviate' spell.

The part of young Moaning Myrtle is played by Shirley Henderson, who is a 37 year old woman.

Jason Isaacs (who played Lucius Malfoy) liked the silver-tipped cane that he carries around so much that he tried to sneak it off the set, and was most displeased when he was caught.

Fourteen Ford Anglias were destroyed to create the Harry and Ron driving sequence.

In Philosopher's Stone (2001), Hagrid taps the bricks behind the Leaky Cauldron and as the bricks start to rearrange themselves creating the doorway, pause the film for the quick glance at the old green sign that reads, "Flourish & Blotts" which hangs over the corner bookstore. This is the bookstore in Chamber of Secrets that Harry and Hagrid pass on their way out of Knockturn Alley, but the old green sign obviously reads something else as it is not the setting for Flourish & Blotts in this film.

Christian Coulson landed the role as Tom Riddle, even though he was 23 at the time of filming, and exceeded the 15-17 age group set for auditions. Ironically, in 2007 when Coulson expressed an interest in returning for Half-Blood Prince, director David Yates declined, feeling that Coulson was "too old" to play the role again.

In the movie trailer, there is a scene where Ron and Harry are dragging Crabbe and Goyle into the broom closet. It didn't make the movie and shows up as a deleted scene on the DVD.

In Asian releases of the film, the Chinese subtitles for 'Draco Malfoy' has a double meaning. It is pronounced as 'zhuai ge', which also means 'arrogant guy'.

Moaning Myrtle is played by Shirley Henderson who was also in the movie "Bridget Jones' Diary" (2001). Henderson's first appearance in that film is her standing in a bathroom crying, also what Moaning Myrtle does in her part of the film.

Actor Hugh Grant was originally asked to play Gildroy Lockhart, but turned down the offer as he had already started working on another film.

A cinema manager in Norway said that the movie was making younger kids throw up in the theater. The reason was they were eating sweets and watching Ron burp up slugs at the same time.

As Hagrid and Harry are walking away from Knockturn Alley, look closely in one of the shop windows in the background - there's the entire set of the Harry Potter books.

The reason Susan Bones is in so many scenes with the Gryffindors, although she was sorted into Hufflepuff, is because the actress is Eleanor Columbus, the daughter of director Chris Columbus. Two of his other children are also in the movie.

In the first Quidditch Match (Gryffindor VS. Slytherin), the Slytherin team scores first. After they score, they all form up into the Hawkshead Attacking Formation mentioned in the 4th book.

When Harry uses the Floo powder and ends up in Knockturn Ally, the first thing he looks at is a necklace. If you read the 6th book( you will know that Katie Bell (a Gryffindor) touches a necklace from Knockturn Ally. The same one from this movie.