Ade Barkah has found some security issues associated to Google Docs:
First, apparently when you embed an image in a protected document it gets uploaded to a Google server where people you’ve not given access to the file can still see and download it. Check this link.
Second, it appears that if you share a document carrying a diagram - a feature Google introduced yesterday - with anyone, this person will be able to view any version of any diagram that has been embedded in the document.
Third, if you take away the permission for another person to access your documents, they could in some cases still be able to get to them later without your knowledge.