Sometimes I click threads open and read through them and then add my comment. When I do this, I get a single line that says, "Unknown or expired link."
If I refresh, the thread appears and I can post my link.
This problem stems from a limitation of the way HN maintains state. An earlier post with some links to the problem code is here: http://news.ycombinator.com/item?id=163755
This is a well known problem pg hasn't gotten around to fixing. See YC: "unknown or expired link" -- fix it already
(790 days ago)
http://news.ycombinator.com/item?id=28944
