Freeman Stephen wrote:Its to do with your cookie timing out, or your internet connection resetting.
Most of the time it's a cookie time-out, they seem to do this after about an hour, well in Firefox they do anyway. In this case, with Firefox, all you have to do is Log in again, then back-page to your post and resubmit as Kazz recommends. That is if you don't close your Browser, because then it's gone. If you need to close your browser, then copying the text to your clipboard is highly recommended.
You're not alone, it catches me out too

Just because it's Legal doesn't make it Lawful, however, what is Lawful must always be Legal.