Browsing as Anonymout Coward, you see something that you want to post a comment to. You start typing your comment, then decide you'd like to post it logged in. So you log in, but now that you're logged in, and instead of being able to resume composing your comment, you are redirected to the home page, and your comment draft is gone.
Logging in from a comment form should preserve your place in both the story and comment you are commenting on, and preserve the text you had entered thus far.
Log in to post a comment.