I hope everyone takes what they read on the internet w/ a grain of salt.
http://www.greenies.com/faqs/details.cfm?ID=146
It says on their website, that greenies are "highly digestible", not 100% digestible.
While most of the time, this is not a problem...as most non-digestible things pass out of the rectum. The particular dog in question may have had some blockage issues, which when compiled w/ the Greenie created a large issue.