Problem? I thought this was a feature. It happens when the user does not interact with a page for some time.
I thought HN has it so that the users are forced to refresh the screen and get the latest news.
Am I wrong here?
Ha. Never thought of that. But no, it's not a feature in the sense of something that was designed with this goal in mind. It's a way of not having to design and implement a whole url scheme, and instead being able to easily run arbitrary code in response to a link being clicked. This architecture, however, has the undesirable (expect apparently to you) side-effect of occasional "unknown or expired link"s.