A possibly related issue is that I occasionally find myself logged in as someone else when I load the front page (ie some other user's name/karma are displayed in the top right). This goes away after refreshing or loading any other link (so I can't post as someone else, for example).
Plus I get that "unknown or expired link" error sometimes when trying to post (including when trying to post this message). This seems to occur when clicking on a thread from the front page loads an out-of-date version of the thread, and can be fixed by force-refreshing the thread page.
Plus I get that "unknown or expired link" error sometimes when trying to post (including when trying to post this message). This seems to occur when clicking on a thread from the front page loads an out-of-date version of the thread, and can be fixed by force-refreshing the thread page.