Matthew P. Thomas: "Half a second before he clicked, I realized what he was doing, but I regret to say I was struck immobile by horror and disbelief."
I've had this happen to me multiple times usually while using my Flash editor. If you unfocus the browser window while using the editor then refocus the window, the editor will lose focus and you'll have to click within the editor to regain focus. If however you hit the backspace key because you still see the flashing cursor within the editor your browswer will back up a page and lose your changes upon returning as the editor was not really focused. This is probably a bug that might be fixable within the confines of the current Flash player but it illustrates that it would help to retain a documents state upon an unexpected exit of the page.
An addition I'll make to the editor's wanted feature list is frequent saving most likely to a local session variable that will re-populate the lost document if it hasn't been posted but was lost.
Ideally it would be best to create the document then have it continually saving thus removing the idea of the "Save" button altogether. Also inspired in part by mpt's When good interfaces go crufty.