It's the "something irrelevants" that get you. Every time. Below, my list of the most common "something irrelavants" that have caused me to do an otherwise unintended reboot. (Numbered just because I like lists with numbers in them, not in any particular order.)
1: Oh. I thought that was the printer power cable.
2: Motherboard upgrade.
3: Spend 20 minutes on a Windows NT box. Switch over to one of my OS/2 boxes. Decide, for some reason, to inspect the list of running processes. Press Control-alt-del. Damn!@# (I should explain, for the youngsters, that once upon a time, CAD always meant "restart right now, and don't argue". I firmly believe that the only reason Microsoft chose to break this convention when they introdiced Windows NT (apart from the fact that they just like breaking perfectly good conventions anyway) was because if they could get people into the habit of pressing CAD to logon, then they would be constantly rebooting their OS/2 systems. There are any number of third-party patches, hacks and utilities to remove the sudden death CAD, but I don't bother with them. Usually, I just remember which OS I am using. Except when I forget.)
4: I'll just reach down and press the floppy eject button. Damn! That was the reset switch!
5: Ran out of RAM, shipment didn't arrive on time, so I walk round the office downgrading all my own machines.
6: There is a customer system at the front of the workshop job queue. "David", I say, "I wouldn't plug that machine in if I were you. I think there is something wrong with the ...." (phone goes dead, lights go out, A/C system stops, alarm system starts beeping, the sad sound of 17 hard drives slowly spinning down) "..... power supply".
7: Putting the main server, which lives in a cupboard, too close to the front. Then, when you are finished doing that little job (slipping a CD into the drive, maybe) you close the cupboard door and it triggers the ATX power switch.
8: RAM shipment finally arrived, so I walk round the office with a handful of DIMMs, upgrading all our machines again.
9: Cat stood on the protruding double adaptor that leads to the extension cord that leads to the powerboad that supplies the juice to my home computer. Yes, really! It's happened twice now.
10: CAD, CAD, CAD. Why won't you shut down? Oh bugger! Wrong keyboard.