(1) open the 'reply' link in a new tab
(2) compose the reply
(3) submit, getting the 'unknown of expired link' error
(4) go back -- you still have your comment, but...
(5) hit reload, figuring that will refresh your reply form's fnid validity -- after all, this works when commenting at an article's top level
(6) get the "unknown or expired link" error now on the reload, with no place to go further "back" to, and "forward" just leading to the same error. Your comment is unrecoverably lost.
I'm now in the habit of a textarea "select-all, copy" before ever hitting a submit button at News.YC. Thus, I can reclick a path from a fnid-less URL to a new reply box if necessary. But that's a pretty user-hostile workaround to expect of people.