People might manually change critical config files in servers occasionally. For example, /etc/hosts, /etc/hostname, etc.
As an experienced operator, you will remember to backup, before making any changes. Right? What would you do? cp /etc/hosts /etc/hosts.bak.
But is that good enough?
Rule #1: Manual Changes Are Troublesome.
Try our best to avoid them. Use configuration management tools if possible. Chef, Puppet, Ansible, whatever. (Related Reading: Automatically Track All Change History)
Back to the earth. You might still want to change files directly in some cases. Yes, I totally understand. Quick fix in non-critical envs. Test some stuff in local dev envs.