I think this occurs whenever AJAX quick reply is used to post a new comment, but the user's comment gets added to the moderation queue. Thus the assumption that AJAX quick reply would ALWAYS return at least one post is flawed, and this error is fixed in the next release by removing that assumption.
However, I'm not sure if a user whose comment will become moderated should even have access to quick reply. I'm not sure what the standard is for this situation in XenForo.