Last night, I managed to lock myself out of a friend's colocated server that's running Centos / cPanel. I changed the SSHd port from 22 to something non-standard, and then restarted the service without opening up that port in the firewall. I've made this change on every server I've setup or managed for the last 6 or 7 years; I just forgot that step this time... oops.
I can get into cPanel/WHM, but can't seem to figure out a way to fix this. I can't set up a cron job in WHM as root, can't ftp in as root... Short of hiring a pair of remote hands (I'm in Las Vegas, my friend is in San Diego and the server is in Los Angeles), I'm not seeing a way to fix this.
Exactly why the servers I purchase have IPMI where I have complete control over the hardware. I know when I offer dedi's to my clients it will be that same hardware, that way if they goof up they have power and local console over IP access and can reload and get root access locally.
Supermicro systems usually have this feature. IPMI KVM w/Virtual Media is the best server management I've experienced.