Ben Treynor, VP Engineering and Site Reliability, dedicated an official Gmail blog post to describe the issue and its cause:
I know what some of you are thinking: how could this happen if we have multiple copies of your data, in multiple data centers? Well, in some rare instances software bugs can affect several copies of the data. That’s what happened here.How many of us are using free services such as Google's Gmail to store private (and sometime important) data? I know I am. I use Gmail and a bunch of other free services regularly (Dropbox, Google Docs) as I work on my new book.
I think its high time for me to consider my backup options, and my entire work-flow. Since I'm working on my iPad, every sync to iTunes should backup my Pages documents to my laptop. But my syncs are not that frequent, so I also use Dropbox as an external, free, Cloud Based, storage. So I might one day log in only to find my entire storage's empty.
As I said, its high time for me to consider my backup options.