One funny Harry Potter fanfiction story is about Fred and George trying to turn the Hogwarts stairs into a giant slide. They accidentally make all the stairs move in different directions at once, causing chaos as students try to get to their classes. Teachers like McGonagall are left chasing after students who are sliding all over the place.
There's a story where Neville accidentally transfigures his toad into a huge, bouncing ball. The toad - now ball - starts bouncing around the Gryffindor common room, knocking over furniture and chasing after Scabbers. Everyone is running around trying to catch it, and Neville is frantically trying to reverse the spell while laughing nervously.
In a rather humorous fanfic, Hagrid decides to start a knitting club for magical creatures. He gets Fang, some of the Hippogriffs, and a few house - elves involved. The Hippogriffs keep getting their knitting needles tangled in their feathers, and the house - elves are constantly correcting everyone's stitches. It's a very odd but hilarious sight to behold.
One funny story is when Neville accidentally used a spell that made his ears grow really large during a Defense Against the Dark Arts class. Everyone couldn't stop laughing, and even Professor McGonagall had a hard time keeping a straight face. It was so out of the blue and Neville's shocked expression just added to the hilarity.
One funny Tumblr story was about a fan who tried to make Butterbeer at home. They followed an online recipe but ended up with something that looked like green slime instead of the delicious drink from the wizarding world. Another was a story of someone trying to do the Patronus charm like in the books and movies, but their cat just stared at them like they were crazy. And there was also a story of a fan who went to a Harry Potter convention dressed as a house - elf and accidentally tripped over their long ears and landed in a pile of Bertie Bott's Every Flavour Beans.
One funny short story could be about Fred and George Weasley's latest prank at Hogwarts. They turned all the Slytherin's robes into bright pink ones overnight. The Slytherins were horrified as they walked into the Great Hall in the morning, much to the amusement of the other houses.
There's also 'Harry Potter and the Misadventures of the Marauders'. It imagines James Potter and his friends getting into all sorts of comical trouble at Hogwarts. For example, they accidentally turn themselves into animals during a botched Transfiguration prank.
Harry Redknapp once told a story about a player who misheard his instructions during a game. He said 'go left' but the player thought he said 'go long'. So the player kicked the ball as far as he could in the wrong direction. Redknapp just laughed it off after the game and it became a great anecdote in the team.
One great funny Harry Potter fanfiction is 'A Very Potter Musical'. It takes the beloved characters and puts them in hilarious and unexpected situations. The characters' personalities are exaggerated in a comical way, like Ron being even more food - obsessed and Hermione being super - intense about schoolwork. Another is 'The Unspeakable Adventures of Harry Potter' which has a lot of wacky magical mishaps that Harry and his friends get into, making it a really fun read.
There was the time when Harry put on the Invisibility Cloak and tried to sneak into the restricted section of the library. But he accidentally bumped into a bookshelf and made a huge noise, causing Madam Pince to come running. He had to quickly run away while still under the cloak, and almost ran into Filch on his way out.
One funny idea could be a story where the Marauders accidentally turn themselves into cats and have to deal with life in Hogwarts as felines. They'd be chasing mice instead of playing pranks on Snape for a while, and it would be hilarious to see them trying to communicate with the other students while being cats.
The humor in Harry Potter fanfiction can come from the fact that it allows for more creative liberties. Fans can explore different relationships between characters in a light - hearted manner. For example, they might write about Hermione trying to teach Snape how to dance, which would be both absurd and funny considering their personalities in the original story. Also, the use of magic in comical scenarios, like using a spell to make Dumbledore's beard change colors every time he sneezes, adds to the hilarity.
A really entertaining one is where Peeves teams up with the Weasley twins. They play pranks on the Slytherins by making all their green robes turn bright pink. The Slytherins are furious, but can't seem to catch the three tricksters. Another funny story is when Harry tries to use a spell to make his hair look nice for a date, but instead ends up with a head full of snakes like Medusa. His friends can't stop laughing when they see him.