Always, always keep a copy of every submission and revision that you send to publishers. A great way to do this is to e-mail your Gmail or similiar webmail account with the file as an attachment. That way the story is "out there", even if your computer crashes.
You already know this, of course. I already knew this too. Yet, I still managed to stuff this up. For the full story, see my post titled How Embarrassing at my Surge Bin blog.
The moral of the story? We can all slip up by not doing something we know we should do every time. There is no real cure to the disease of being human. I can only say that I know that I'll be more dilligent, and I hope that you can learn from my mistake and be more dilligent too.