You must be new here. Links on the site fall off the stack eventually--but when you reload the page, new ones are made. It's a "feature" of the architecture pg used to build the site (continuations based).
So, if you sit idle on any of the comment pages for more than a few minutes it becomes unavailable. You have to start over on the main page and dig your way back down. I imagine this is not a fundamental flaw of a continuations-based design, since Seaside seems to cope with it OK (I guess?)...merely something that pg, or someone else, has not added the code to handle yet.