Have you ever have a mistake save a Office File like Microsoft Word or Excel, and then cannot be Undo or Rollback?
If you using Dropbox.com and Placed your file at Dropbox Folder, even if it' s a Free Version, you can get the previous saved version. You only need to access the web interface and then right click the file, find previous version tab.
You must place your File at Dropbox Folder to have this Facility, and maybe period is Limited. Good Luck.