There is always the - Is your computer plugged in. I have actually caught that one more than once. It was especially bad when i had to charge a travel fee and minimum charge to go on site to fix it after the lady swore up and down that it was plugged in and everything was setup properly. I think the janitor had used the outlet for a vacume or something.
I had that 3 times in two weeks, one location had multiple things unplugged ... and had a computer plugged into a 10 base T hub and wondering why it was running so slow ...