If you preview a post you're writing, but someone else has posted in the topic since then, you get the usual 'A new reply has been added' and the new post is added to the Topic Summary pile. However, if you preview it after that, the new ninja-post is added again, and this repeats every time you hit Preview. If you then hit Post, it displays properly, but you need to hit post a second time to actually get it to work.
Also we Librarians should've been staff members a long time ago- wait, we're not? :<