I work in an electronics store (been there since 2004) that deals with a bunch of odd low-voltage stuff. I have seen quite the range. Most of the customers are contractors; but even then there are some that scare me.
More than once an electrician was called on to run and terminate a wire for a computer network... and they came to me for lessons on how to install the connectors.
The really worrisome guys are the ones who have no spark of intelligence in their eyes, and need to have something demonstrated to them ten times before they grasp the first principle... how the HELL did they even get that job in the first place?
The flipside of that are the "Neckless Nobodies" who can't tell which end of the AA battery is positive or negative. Even with illustrations and labels.