Hmmm. I don't think it has anything to do with what *you* aren't doing. Sounds like you found a bug.
Was there anything else that seemed odd?
I'm assuming you didn't get it right the third time.
Could you try to edit a post on a different thread (e.g. this one) and see whether that works?
Also, what browser and OS (Mac or Windows) were you using when this happened?
[edit] Seeing whether I can edit a post.
[edit 2] Yes, it seems that I can.