This one is on my last nerve.
1) Click on post.
2) Think about writing up a response.
3) Go do something.
4) Wait ten minutes.
5) Write up a response and hit "submit."
6) "Unknown or expired link."
7) roll back, roll forward, and hope you can ctrl-c what you wrote rather than have it vanish into the aether.
8) refresh link.
9) ctrl v if you're lucky.
10) make the comment you wanted to make in step (5).
If you're going to time it out, could you write what I wrote into cache and give me a place to resubmit? I'm sure it has something to do with your structure changing when there's a comment, but it's like playing raquetball in a minefield.