I think this is a feature, and not a bug.
/sarcasm
-Edited to be more accurate of the inside of my head/thought process
Think of an always up-to-date list divided in two pages, clicking on the second page would take you to a list of links that you might already have seen on the first page. Here, the second page is kind of "frozen" in time, aka the session and only displays the rest of the links.
I might be wrong though.
What I don't know is why a "news.ycombinator.com/page/2" url design couldn't be handled with a client-side session and cookie and still offers the same feature.