Why does HN Say unknown or expired link when I click "more"?

<a href="http://news.ycombinator.com/item?id=766839" rel="nofollow">http://news.ycombinator.com/item?id=766839</a><p><a href="http://news.ycombinator.com/item?id=163696" rel="nofollow">http://news.ycombinator.com/item?id=163696</a><p><a href="http://news.ycombinator.com/item?id=28944" rel="nofollow">http://news.ycombinator.com/item?id=28944</a>