Message I've got: "It's unlikely that you're having a problem with your Documents List.
Rather, you might be having trouble with an individual doc or file.
Please post in the Google Docs Help Forum to resolve your issue.
Fortunately I've lost only approx. 40 minutes of work. Because the data I've entered still was on paper, and I didn't trash it :)
Interesting to note that after several days I've got similar problem with other opened spreadsheet, but I've manage to recover 2hours old version of it. Very annoying errors from Google Docs. And now I don't feel as comfortable with Google Docs as before.