Funny / Harry Potter and the Nightmares of Futures Past
Harry contemplating how to get Sirius out of Azkaban: "Harry was tempted for a moment to just tell Fred and George that one of their heroes was locked up in Azkaban and then stand back. On the other hand, the Ministry might need that prison again some day."
When Snape begins to step up his use of Legilimency during Harry's second year, Ginny and Luna trick him into using it on Luna, giving Snape a bit of a headache.
Fred and George decided to prank Harry and Ginny by sending the former the latter's infamous Valentine poem. Harry and Ginny's payback? A Valentine in their name to Melissa Bulstrode, a Slytherin Prefect known for having decked a unicorn, resulting in them gaining a scary Abhorrent Admirer with Super Strength.
The best part? Melissa saw through it, but figured out they deserved it for something and decided to have fun.
From Chapter 27, "This ponce is Voldemort?" What makes this even more hilarious is that it's Ron who says it.
From Chapter 34:
Hermione: What if you get struck by lightning? Ron: Hope the broom doesn't explode between your legs?
Luna's Brutal Honesty about her attraction to Neville is made of this. For example, Luna doesn't understand aerial tag.
Luna: What if I want Neville to catch me?
From Chapter 38, Luna produces a Patronus...in the shape of a duck-billed platypus. When Ron congratulates her on having an imaginary animal for a Patronus, Hermione tells him that no, it's actually a real animal. He doesn't believe a word of it. Luna is completely straight faced, but Harry laughs so hard he almost passes out.
In chapter 39 Harry decides to make a joke on Ron's supposed status as Heir of Gryffindor and Colin's belief no teacher would dare giving him a failing grade:
Harry *impersonating McGonagall*: "Mister Weasley, in light of your... heritage... I will accept nothing less than your best work in my class. Is that understood?"
In chapter 41, Barty Crouch Jr steals Harry's wand and tries to kill him. Unfortunately for him, what Harry had in his pocket was one of the twins' fake wands. The image of Barty Crouch Jr wielding a rubber chicken like a wand becomes quite the hilarious scene.
In the aftermath of the attack, Harry amusedly sees that the Aurors are very carefully stepping around Ron. Who gets recognized by the Bulgarian Aurors as "sword-boy".
When Sirius learns Percy was the one to stun Barty Crouch Sr, he proceeds to give him (Percy) a thorough tongue bath.
In Chapter 42, Moody has become the new Professor for Defence Against the Dark Arts and has a special first lesson, consisting of hiding in a corner and attacking the students when the bell rings. As soon as she enters the classroom, Luna goes straight to where he is hiding and says "good morning": she catches him because his Disillusionment Charm does not cover the smell of the pickled herrings he had for breakfast.