This is somewhat annoying. If you have a page with 5 or more interesting links and then after some minutes want to continue, it breaks the xp on ycombinator entirely. I usually leave the page. Why in any hell don't you even just link to the start (or include the header bar)?
I bet it's because of caching the query, to avoid post sliding over page indexes after time, or whatever you thought with that non-session and non-user token. But. It. Is. Somewhat annoying, to cut the ?x=whatever just to return to news.ycombinator.com.
Use a timestamp to refresh the query or go back to use old school bad page indexes, but please fix something, at least the error message. Please!