The page seems completely clear and unambiguous, but people clearly confuse the link to reply to the journal post with the link to reply to the last comment on the page. During the year I've been using LJ, I've seen numerous posts where someone has replied to a comment intending to reply to the journal entry, or vice versa. Rather to my suprise, I just realised I've done it myself in a recent thread I posted in.
It's commonplace enough that I'm coming to the inescapable conclusion that there's a UI design problem, but it's obviously fairly subtle.
I assume everyone else has seen this mistake made; anyone care to comment what it is about that page that makes it confusing to use?