Canadian consultant Ade Barkah discovered three security problems with Google Docs, all of which expose information that you might otherwise think is protected.
In his blog Ade details each of the problems. In short, one problem is that images are not protected by the sharing controls; the second problem is that "anyone you shared the document with can view any version of any diagram embedded in the document,"; and equally as bad, if you share a document with someone and then remove their access they might be able to still access the document.
These three problems point out why online Web apps ought not be used for handling sensitive information.
That somewhat elderly adage of the Internet still applies today (after nearly a decade and a half): Do not send any information across the Internet that you don't want the entire world to see.
