On large, complex sites you will often find many "hidden" subdomains that duplicate the content on the main domain. You might even find that the dev server has been indexed (e.g., http://dev.example.com/). Checking for subdomains should be part of every site audit.
A simple Google query will show you a list of subdomains on a site by excluding domains that begin with www:
site:example.com -www.example.com
Change the view to 100 per page and visually scan the SERPs for subdomains. You can quickly convert the view to 100 results per page by appending the text &num=100 to the Google SERPs URL.
Below is an example query that shows the subdomains on Google.com. Note the &num=100 on the end of the URL that tells Google to display 100 results:
http://www.google.com/search?q=site%3Agoogle.com+-www.google.com&num=100
