Since no one (neither Harry before remembering the gaze of death line, nor the aurors confronted with Bellatix' inexplicable disappearance, nor the designers of Askaban) expected it to hide you from dementors being "a kickass invisibility cloak" is evidently not enough to make the fact that it does completely unsurprising, so naturally that fact is evidence for any hypothesis that does an even better job of explaining it.
There's only one true invisibility cloak, and it's not enough to get someone out on its own. What reason to we have to suppose that the aurors or the designers of Azkaban ever bothered to consider the question of whether it would hide someone from dementors? The aurors were surprised that Bellatrix had vanished from the senses of the dementors, but this is clearly an unusual occurrence that should not arise within ordinary experience, so we should not be surprised at their surprise. They do not have "someone sneaked in with the true invisibility cloak...
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.