Pretty much everyone has been in. From 2001-2011 it was basically a rite of passage: if you were a British actor, you would graduate from RADA, waft along the halls of Hogwarts at least once, then apparate away.
So it's small wonder that we didn't remember that these performers all helped bring JK Rowling's Wizarding World to the big screen.
1. Michelle Fairley – Mrs Granger
You know her best from your haunted dreams after the horror of' Red Wedding, but before GoT, Catelyn Stark herself had a brief appearance as Hermione's unnamed mum in Harry Potter and the Deathly Hallows: Part 1, replacing The Chamber of Secrets' Heather Bleasdale.
2. Domhnall Gleeson – Bill Weasley
Theand actor made the most of his red locks as the eldest Weasley child, who shows up in Harry Potter and the Deathly Hallows Part 1 just in time to have his wedding to Fleur Delacour ruined by the Death Eaters. He returned for Part 2.
Actor Richard Fish portrayed Bill in a picture seen in The Prisoner of Azkaban.
3. Jesy Nelson – Hogwarts pupil
Okay, so we can't blame you for missing this one. We couldn't even find a picture of Little Mix's Jesy in the film, but she claims to have been an extra in Harry Potter and the Goblet of Fire's Yule Ball scene.
4. Frank Dillane – Tom Riddle
Thestar played the particularly creepy young Voldemort, who flirts the secret of Horcruxes out of his potions master, Professor Slughorn, in Harry Potter and the Half-Blood Prince.
5. Jarvis Cocker – Myron Wagtail
The lead singer of the band The Weird Sisters—who played at The Yule Ball—was real-life frontman Jarvis Cocker of Pulp fame. The rest of the band comprised of Radiohead's Jonny Greenwood and Philip Selway, All Seeing I's Jason Buckle, Add N to (X)'s Steven Claydon and Pulp's Steve Mackey.
6. Ian Brown – Leaky Cauldron patron
The Weird Sisters weren't the only musicians to sneak into Harry Potter. The Stone Roses' Ian Brown can be spotted in the foreground of Harry Potter and the Prisoner of Azkaban, reading Stephen Hawking's A Brief History of Time at a table in the Leaky Cauldron.
7. Freddie Stroma – Cormac McLaggen
Stroma appeared in the final three Harry Potter movies as Gryffindor and Slug Club member Cormac McLaggen, who Hermione invites to Slughorn's Christmas party (and lives to regret it). He also played DJ Luke in, obnoxious backpacker Ben in and—most notably—Dickon Tarly in season six (but not seven) of Game of Thrones.
8. Julie Christie – Madam Rosmerta
Doctor Zhivago legend Julie Christie snuck into The Prison of Azkaban as the landlady of The Three Broomsticks in Hogsmead. The character didn't reappear in later movies, so neither did Christie.
9. Paul Whitehouse – Sir Cadogan
The Fast Show comedian Paul Whitehouse played Sir Cadogan, the painting who guarded the door to Gryffindor Tower after Sirius Black scared off the Fat Lady. You might not have recognised him under the voluminous moustache.
10. Verne Troyer – Griphook
Best known (somewhat depressingly) as Mini Me in the Austin Powers films, Verne Troyer played the goblin and Gringotts employee Griphook in Harry Potter and the Philosopher's Stone.
Confusingly, he was dubbed by Warwick Davis, who also played Professor Flitwick and ended up portraying Griphook outright in the two Deathly Hallows movies.
11. Zoë Wanamaker – Madam Hooch
The My Family actress played Hogwarts flying instructor Rolanda Hooch in Harry Potter and the Philosopher's Stone, where director Chris Columbus seemed to have an unusual obsession with her coloured contacts.
12. Zoella - Hogwarts pupil
Zoella is listed as "Girl in Potion Class (uncredited)" on her IMDB page, harking back to her role as a Hogwarts pupil. In a blog post dating back to 2009, Zoella talks about her starring role, and explained how she got the part in the first place.
"I was 10 when they first started filming for the first film, and my primary school is in the village they used. So they came & asked everyone in my year (which was probably about 7 children) if we wanted to be extras. & of course we said yes. I was in hufflepuff :)
"The first year, we were used in Snape's potion class scene. I am not seen in the film annoyingly, but its nice to know I was there. We also got to mingle with the cast, we watched spider man with them. Haha & Devon Murray (who plays Seamus Finnigan) asked me out..haha. I said no. I was 10 years old and boys scared me."