A PEBKAC error is often an intermittent error that occurs even though absolutely everything on the system checks out.  But no matter how well the system is running, or how perfect you do your job, whatever it is just does NOT want to work correctly.

So usually after spending a lengthy amount of time on the phone with the user, and deciding that you just going to have to go on-site to see what the problem is yourself, you find out that the problem isn't the system after all.

Problem Exists Between Keyboard And Chair.

Computers are stupid.  They do exactly, and only, what you tell them to do.