I often leave HN tabs open, then start writing a sketch of a comment, then continue later, and notice the link has of course expired. This means going back, reloading the page, and re-pasting the comment. Irritating but bearable.
However, this doesn't work if my noprocrast period gets switched on while I'm reading other articles. I know noprocrast allows submissions, but it needs me to submit with the old link. And the link never gets too old.
I bump into "Unknown or expired link" almost every day.