We host a number of blogs, and one issue we keep seeing is a user making a post to their blog, hitting publish, and simply getting kicked back out to the front page of the blog without the post being saved. This is happening with all versions on the server from 2.1 to 2.8.
It would not seem to be a session/cookie issue as the users are still logged in to the blog even after being dumped to the front page.
edit: left out a big part... the problem is not resolved by deactivating plugins, and has happened to sites with no plugins at all. Upgrading to the current version usually fixes the problem, but not always, and sometimes happens with the latest versions as well.