I agree that you could read it like that, but I'd have thought that if it was something immediate like that, we'd have seen Harry realize and acknowledge it to himself. There doesn't seem much point in leaving it a bit mysterious if that's all it meant.
There's an abrupt scene change, after which Harry is sitting in McGonagall's office. Now, his conversation with McGonagall is evidence for Harry being dim about that question (which is not that unlikely) or it being foreshadowing, possibly for this (which is not that unlikely). (McGonagall would also have to not point out "hey, maybe it was that you forgot my rule", which makes the first somewhat less likely.)
- 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.