for some reason that looked accidental.
You must mean: for some reason that didn't look accidental at all.
A Death Eater sent to gather Harry's blood would work quite well, since it fits in and supports quite well any model of reality in which the prison break involved Voldemort and not Harry (the two options being complementary in Dumbledore's thinking).
But if an unrelated accident were to happen, all of the Order of the Phoenix would ask themselves "how high are the chances that Harry randomly ends up incapacitated during the exact evening in which he is to be put to a critical test of good behaviour?", answer "pretty damn low" and treat it as a 95% chance of guilt.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
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: