"What's that one? Ah, it's the fan heater isn't it? I'm not using that."
[Unplugs the fan heater]
Half a second later: "Why has the server cupboard gone quiet?"
Bad: I unplugged one of my office servers, the one that handles incoming email and web and various other external services.
Worse: The (admittedly rather old) system disk decided it'd had enough and refused to spin back up upon reapplying the power. Always a risk when you power down a machine that's been running constantly for years.
Annoying: I accidentally breathed in loads of dust when I pulled the disk caddies out (I'm allergic to dust).
Good: I appear to have a spare blank disk.
Better: All the data is safely stored on a separate RAID 5 file server that is still running fine.
Great: I also have a full nightly backup of the dead disk stored on the other server.
Fantastic: The disk that I thought was a spare blank was actually a RAID 1 mirror of the dead one. All I had to do was install Grub on it to get it to boot.