If you wait too long I think that your login session times out. I've had it happen to me that if I leave the comp for an hour or so with an open listing, when I come back to continue it has signed out.
But if it does that just click "back" in your browser, then copy the description, and title & category # if you wish. Paste them into wordpad and start a new listing then just paste from wordpad into your new listing.