It has happened to me, too, maybe five times (but then I post a LOT)...and I believe after I received that msg., as well, about someone having posted while I was composing my post.  Sometimes I "time out"...like in the beginning where you put your name and password in and it tells you  you'll be "on" for 60 mins...I notice I have to go enter those words again before I can go forward in CSC...sometimes I have lost a long post for good (and that's frustrating and sometimes I just don't have time to bother to redo it) and sometimes it's a short one; I never repeat it just figure I did my best, it's gone, that's it.