This error is a minor tax on carefully worded, carefully considered posts. I've lost posts following this error due to back button/refresh mishaps. I could post, then edit, but then people are voting and replying to content that is changing.
(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.
Please do something about this, it was a minor annoyance before, but now it turned into a pretty major headache.
Also, the 'Preview' button would be very nice to have. I know there's a delay setting, but that's not it. I want an ability to privately preview what I've wrote, before posting anything.