If you get the following error message from Google Documents, it means that your document is GONE.
Of course. you get the message when you try to (re)open it from the main document list.
This is maybe why you don't want to enable the new Google Documents format (see https://docs.google.com/support/bin/answer.py?answer=176442).
The bad news is that Google Docs has just encountered an error.The good news is that you've helped us find a bug, which we are now looking into.
We apologize for any inconvenience this has caused you.
In the meantime, if you'd like updates on this and other issues, try visiting our Google Docs Help Group: http://groups.google.com/group/GoogleDocs