I generally have hacker news open in the background throughout the day. If I leave it for a while I inevitably get "Unknown or expired link" when next clicking on the "more" link at the bottom.
For example, (I assume this link will give the same result for everyone) http://news.ycombinator.com/x?fnid=x4ipaJEME2
Why do the links expire? I'm sure there's a good back end reason but as a user it seems weird that the site dies if you ignore it for any length of time. Its like a Tamagotchi.