Testing is important. Illustrating this, once the latest Ubuntu mcollective-plugins-facts-facter package is installed, it can't be removed without manual intervention. The postrm script contains the following sed command:
sed -i -e "s/^factsource.*/factsource = yaml\nplugin.yaml = /etc/mcollective/facts.yaml/" /etc/mcollective/server.cfg
There is no way that this can run successfully, because those un-escaped "/" characters in the path "/etc/mcollective/facts.yam" mean something to sed, breaking it. This failure is caught by the package system, leaving the package in a broken state. Something which would have been quite clear to the person writing the package if they had ever tested removing it.
BTW, to fix this, edit /var/lib/dpkg/info/mcollective-plugins-facts-facter.postrm and change the above line to:
sed -i -e "s/^factsource.*/factsource = yaml\nplugin.yaml = \/etc\/mcollective\/facts.yaml/" /etc/mcollective/server.cfg
You can now successfully remove the package.