This has been discussed many, <i>many</i> times. Try searching for it in the search box at the bottom of each page.<p>However, short answer:<p>The ordering of items on a page can change, so when you open the page the ordering is created, and continuation pages are generated by closures in the code. These closures are garbage collected after a while, so if you don't access one in a timely fashion, by the time you ask for it, it might no longer be there.<p>Garbage collections will happen more often when the system is more heavily loaded, so the time limit is effectively shorter.<p>Now, here is some background reading:<p><a href="https://hn.algolia.com/?q=unknown+expired#!/story/forever/0/unknown%20expired" rel="nofollow">https://hn.algolia.com/?q=unknown+expired#!/story/forever/0/...</a><p>You could have looked that up for yourself. Did you?
The overall ranking is updated every few minutes. Each front-page has a different `?fnid=xiJqLAhmD8VrsD2p21gUKn` parameter to ensure page 2 displays that ranking. Clicking the `more` link after HN has updated shows you the expired link message.<p>I guess this means they don't have to store all the old rankings, but it does seem curious not to allow people to view old rankings.