If you look closely at the scene in The Prisoner of Azkaban, Buckbeak does a poo on the floor just before Harry approaches him. He couldn't hold it in!
For a film series that uses a lot of dragons it's nice to see that none of them were harmed during the making of Harry Potter and the Goblet of Fire. You can see the little in-joke hidden in the credits right near the end.
Professor Remus Lupin was a half-blood wizard who was afflicted with lycanthropy during his childhood. Lupin and and lycanthropy means turning into a werewolf, which of course he does in Harry Potter and the Prisoner of Azkaban.
If you look closely in the background of the duel scene of Harry Potter and Chamber of Secrets, you can see the moment when a camera person gets caught in the main film. There are a few mistakes in the Harry Potter films but this one is pretty noticeable!
The scene in Harry Potter and the Deathly Hallows Part 1, in which 6 other people have to disguise themselves as wizard Harry, took actor Daniel Radcliffe 90 tries to get it right. He had to act every part himself!
Harry's feet can't be seen in the Yule Ball scene of Goblet of Fire as actor Daniel Radcliffe didn't have time for dancing lessons in real life. Oops! The rest of the cast can be seen dancing away.
Seven is used as a powerful magic number in Harry Potter world. There's the seven Harry's scene, seven books, seven snaked on the chamber of secrets door, and seven players in a Quidditch match. Can you spot any other sevens in the Harry Potter films?
It took a photoshoot of 40 kittens to create Professor Delores Umbridge's moving kitten plates in her office. We want some!
Rupert Grint, who plays Ron Weasley, has arachnophobia, which means he is REALLY scared of spiders in real life. So the Aragog scene in The Chamber of Secrets must have been especially scary for him! You can tell by his face.
Oculus repairo! In real life Daniel Radcliffe got through 160 pairs of glasses across all 8 Harry Potter films. If only that repair spell worked in real life.