Ask HN: What's up with this "unknown or expired link" on HN?

So a few minutes pass by when I open HN page, and if I press the 'more' link at the bottom, it shows me a blank page with the above mentioned error message. Same happens when I write some comment on any news article and some time goes by before submitting that. Why would HN do that? Why can't you show "more" news items when some time passes by? What gets invalidated in that time? There might be some valid reasons, just that from usability perspective I don't find it quite good ... (Edit: I didn't spend too much time typing this question, and when I pressed submit button it again showed me the same message. I had to go back, refresh the page and submit again)