Grrr. Bloody firebug. I don't use it any more since it has been incorporated into Firefox - but it was still installed.
Long story short, a dozen times now over the last several months I would be composing a long text and my fat fingers slip reaching for the backspace and instead hit F12, which opens firebug. Annoying but no big deal. I close firebug. Then hit the backspace I meant to hit before. But since Firebug was launched the text window no longer has focus. The page does, and a backspace takes you to the previous page - instantly losing all your carefully composed text.
I've just uninstalled Firebug. I'm also seriously considering creating an auto-save or save draft function.