Their behavior is usually predictable and discernable from their appearance. We may never play the violin like a virtuoso, but we understand how the instrument works. Electronic devices are high in cognitive friction. The same button on your microwave oven may set the time, or it may set the intensity. VCR's, with their non-intuitive controls, continue to stymie countless intelligent humans.
Software is VERY high in cognitive friction.
Drag a file from your hard drive to your floppy disk, and you end up with two copies of the file, one on each drive. You can think of other examples where the same action, taken in two different programs, or at two different points within the same program, yield different results. Interaction designers understand how to reduce it. Cooper advocates that interaction designers be brought in at the very start of a software project, before any code is written. Writing software is incredibly difficult work.