Microsoft Office files that can be excluded from your backup routine.
When a file is open in Microsoft Office, it creates a lock file associated with it. The lock file is a hidden temporary file in the same directory with the same name as the main file except that it begins with ~$. Office is supposed to delete it upon closing the file, but sometimes does not. Additionally, if you use an always-vigilant backup tool, these temporary files will get backed up as quickly as they are created.
To keep a clean backup set, I exclude files in my user documents directory beginning with ~$ from my backup tool.