I was just commenting to a co-worker this morning that I have been receiving a lot of expired link errors lately and that HN's session expiration time must be short.

What language is HN written in?

I leave HN open in two tabs all day: one for the newest articles and one for the top articles. Article links to other sites don't expire because they're direct links to an external page. However, most internal links do expire if I haven't refreshed the page in an hour or so.