I was migrating a server and went to change the docroot owner from apache to nginx. I meant to run:

chown -R nginx:nginx ./

What I actually ran was:

chown -R nginx:nginx /

This broke ssh and sudo. I was eventually able to recover it by getting into a recovery and reinstalling every rpm and manually fixing /etc leftovers from there. It was an early cloud server, so I didn't have physical access. This was back when #Rackspace support was worth the premium you paid for it.