This guy comes over to install my cable modem a few weeks back. We start talking about computer stuff, naturally, and he starts telling me about his system. I'll quote him whenever possible:
him: "Yeah, I just got my dual Athlon system goin'."
me: "wow, dual athlon-I didn't realize there were dual athlon motherboards out. what kind of motherboard is it?"
him: "oh, I wouldn't recommend it. (change of subject.) I've got it overclocked to 4GHz."
me: "4Ghz?"
him: "yeah, well 2GHz per Athlon, so four total."
me (trying not to laugh): "wow, you have a Kryotech case or something?"
him: "naw, just some fans."

he then proceeded to:
a) power off my machine halfway through my splash screen.
b) borrowed my tools because he forgot his
c) forgot the PCI network card he needed and put in an ISA NIC.

needless to say, i went over my machine with a fine toothed comb to make sure he didn't leave any screws laying on a card or something. I told myself that I wouldn't look over his shoulder when he was doing the installation. As an "IT Professional," I know it would irritate me, so I kept one eye on his hands and the other on his face for expressions like, "****, hope he didn't see that!"