I started a thread over at our forums named Don't Enable GoogleBot To Delete All Your Pages. In that thread I link to an other forum thread that shows how a Webmaster left open the delete button for GoogleBot to click on. This is what happened in short...
Webmaster rebuilt the site months ago, repopulating all the content into a database driven solution. When the Webmaster published the new site, he/she left the "edit" link open somewhere on the frontend, where GoogleBot can get to it. I assume it was not password protected, and GoogleBot got in and looked around. Since GoogleBot just clicks on links, it clicked on the delete link on every single page of the site, until the whole site was deleted from the database and no other pages or content was available to delete.
Pretty crazy, huh? Yea. But GoogleBot is not to blame for this one.
Forum discussion at Search Engine Roundtable Forums.