Missed "User Domesticus"
You show up at their work station, and start working on their computer trying to figure out what the issue is. While discussing the issue the user is having problems with, they finally admit that it is not the company computer at all, but actually their home computer. I've even had them go as far as bring their computer in.
Now if it's an issue with VPN networking or something, fine, but when it's how do I get onto my email with my Mac? Yeah, not a Mac trained technician, contact your ISP or the Mac store.