One of the worst, most difficult projects I ever worked on was where I was forced into direct interaction with a "customer". She was a nice enough lady in general, and knew her subject area, but she had no technical skills at all. We were paired up to improve a part of the UI and we struggled because she would ask for things that were technically impossible, and get upset when I told her we couldn't do it. It got so bad that she complained to my boss that I wasn't doing my job as her "support person". Fortunately, my boss cleared up her misconception and we finished the project. But her fundamental failure to understand the technical limitations of the hardware and software, as well as not seeing me as a partner, were a huge hurdle.