I have two stories to share... Both are when I worked at TVA.
After upgrading a womans PC, she called my supervisor complaining her PC was making a loud noise after I upgraded it. Needless to say she was very pissed off. I went back to take a look and found her pager set to vibrate going off in her desk drawer.
Another similar situation at TVA... Again, after upgrading another womans PC she called my supervisor saying her PC wouldn't boot up after I upgraded it. It turned out I had turned her PC of when I was done and when she came back she was trying to turn back on using the Monitor power button.
If your into the PC repair business... Don't ever take anything for granted. There ARE some VERY stupid people out there. :P