I think that happens because if you change the title, it'll get a different title and URL sent in the RSS feed, and the reader is going to think it's a different item. There's an RSS tag, <guid>, which is meant to work around this by allowing the feed to specify a unique identifier for each item, but the LW RSS feeds use it incorrectly; it uses the post's URL as its value, and that changes when the post's title changes. I think it would work correctly if it chopped off the post title part and just sent (e.g.) "http://lesswrong.com/lw/2qm/" in the <guid> tag.