In a locked thread, the "reply to this" option on each post is removed. However, if you rate someone in a locked post, the option to reply to that post re-appears. You can bring up the text editor and reply, but then get a white screen when you actually try to hit post (I assume it's an untrapped exception of sorts).
Not a critical issue, but it took me a second to figure out what had happened (yay for not refreshing a thread before trying to post). After a bit of testing, this was what I narrowed the problem down to be.