First, remove the user from the equation. This is harder than it may first appear.
Customer calls me today, says, "I know this computer has this specific problem. It's doing X, Y, and Z. No, I don't use the computer, but I don't have to --- Lazy E computers are a pile of crap, and they always do X, Y, and Z, and I wanted to buy HewPaq anyway. I've been in computers ten years, and I know what the problem is, so don't make me prove it, just send the parts."
I go all peeved-black-lady on him, made him troubleshoot. Customer puts me on speakerphone so he can crack the case and mutter under his breath at the same time. In the middle of his frustration, the end user rolls in to see if the computer is working yet. I introduce myself (cheeky me!), and ask the end user what the computer did. Turns out, the system was really doing A, B, and R. I had the idiot customer, who suddenly doesn't have an extra word to say to me, swap out a cable, and the problem went away. As I'm hanging up, I hear the original customer call me a "stupid b***h."
I cracked up.
Just another day in tech support heaven!