I don't think I hit this before the site came back up. I quickly follow this path:
Click the shortcut to view posts since my last visit
View a topic
Click the back button
I get the following:
You cannot make another search so soon after your last; please try again in a short while.
I then have to wait a mintue and refresh the page. It's only slightly annoying, but if such a thing is convenient it might be fixable by not expiring the search results page immediately. The following headers come down with the search results:
Setting the expire date to a few minutes in the future and maybe removing the no-cache (that one has always been fuzzy to me) should let the old search results come back up when I click the back button.