They learned the phrase from Dumbledore when they gave him the map. The fact that they don't remember the map but imperfectly remember the phrase does support the "poorly done obviation" hypothesis. (either because the obliviator missed removing the phrase or the obliviator intended for them to keep the phrase but accidentally messed it up)
(or, less likely, 'Deligitor Prodi' would not have worked for the twins and the obliviator intentionally altered it so that it would)
A few moments later, Fred and George were handing over the Map to the Headmaster, wincing only slightly at the sacrilege of giving their precious piece of the Hogwarts security system to the person who actually owned it, and the old wizard was frowning at the apparent blankness.
"You've got to say," they explained, "I solemnly swear that I am up to no good -"
"I decline to lie," said the old wizard. He held the Map high and bellowed, "Hear me, Hogwarts! Deligitor prodi!" An instant later the Headmaster was wearing the Sorting Hat, which looked scarily right upon his head, as though Dumbledore had always been waiting for a patchwork pointed hat to complete his existence.
(Fred and George immediately memorized this phrase, just in case it would work for somebody besides the Headmaster, and began trying to think of pranks that would involve the Sorting Hat.)
They learned the phrase from Dumbledore when they gave him the map.
I'd completely forgotten that!
But, y'know, procedural versus declarative memory. It's perfectly possible to remember to say something like "deligitor prodi" when you want the Sorting Hat, while forgetting all sorts of associated details. Er, like I did myself.
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 94. The previous thread has passed 200 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, 19, 20, 21, 22.
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: