Also, never ever treat any system as a backup, if you haven't tested that restore actually works. Do that, and a likelihood of loosing your files would get a lot smaller.
Most of us here are developers. How many of our products are perfect? Always assume something will fail in a new and interesting way in the future. I'm not letting Dropbox off the hook - their product shouldn't do this - but you'll be happier if you treat backups with the same level of redundancy and planning as the rest of your infrastructure.