For (even more) Hilarious in Hindsight: Dumbledore says at the end of Ron's Disease that he's a gay android. Guess what Rowling revealed later? (hint: not the android part).
Some of the entries in "Snape's Diary" are actually rather heartwrenching.
True Art Is Incomprehensible: "The Awakening of the Incorruptible" is almost Dada in its deconstruction of standard dramatic tropes, its rejection of the noise of modernism for the postmodern silence.