Fred and George found out about Rita Skeeter's animagus form, using the Marauder's Map.
They constantly saw her on the map, but never managed to see her in person. The twins investigated, ruling out magical forms of concealment, e.g. invisibility cloak or the disillusionment charm, and discovered her in beetle form.
They blackmailed her, threatening to reveal her secret, and so Skeeter published the article. The twins were obliviated, thus they cannot remember 1) how they got Skeeter to publish, and 2) the Marauder's Map.
Uncertainties:
If the blackmail is enough. I am not sure how the consequences of 1) publishing an article that destroys her reputation after the world realizes it is untrue, and 2) being revealed as an unregistered animagus, compare. I am leaning toward the secret being worse. It also destroys her reputation, as it reveals the source of her stories and her unethical journalistic methods. Plus, she would most likely face legal trouble, though the wizarding world isn't exactly consistent with their laws.
Who obliviated the twins afterward. It may have been Rita Skeeter, as part of the agreement. Or Quirrel obliviated the twins, to prevent the chance of anyone realizing he killed Skeeter.
Rita Skeeter came before Dumbledore borrowed the map, and Fred and George's comments on the glitches were in a context where revealing them to be Rita Skeeter would have made sense (or at least, revealing it by now). It's strongly implied that they just paid someone (my guess is Lockheart) to false-memory charm Skeeter. This points away from the same charmer being behind the Rita Skeeter prank and obliviating the twins' memories of the map; the charmer in Rita Skeeter's case is apparently ridiculously competent (one reason I suspect Lockheart), whereas the twins' recent obliviation was fairly crude.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: