Design defects are bugs. If your design for, say a function to compute factorial, is incorrect, and the code precisely implements the (bad) design, it is a buggy function. For software that exists to serve human users, the facility with which those human users can successfully interact with the software is of primary importance. Really, it is the most important thing, because if the software's function is to allow a human user to do something, and the human user can't readily do that something, then it is a significant failure.
Software outfits that build software for human interaction who understand these things tend to build beautiful and highly usable software. In contrast, outfits that treat the human interaction aspect of their software design as a not as important or meaningful as algorithm design or the coding itself, tend to build difficult to use and unpleasant software. The consequence of this in commercial software is user generated system failures because of mis-understanding of the correct use of the system, or delays in implementation or operation due to difficulties in installation, configuration, and use of the system.
It is the hope of the professional human interaction design community that everyone associated with software development for human users understands the fundamental importance of successful human interaction models. In other fields of design, this is sometimes better understood perhaps because the implementations are physical in nature. For example, to the extent aircraft controls are designed to be used by human pilots, you would pay attention to their placement and actions. For a silly example, putting the "shut engine" button next to the "ask the flight attendant for coffee" button is probably a design that can lead to a rather adverse consequence.
You can get more information about human computer interaction design (aka human factors, aka computer human interaction) from the ACM CHI group, or proceedings from their annual CHI conferences. The IEEE has an analogous effort (with perhaps an even broader mandate) in the Systems, Man, and Cybernetics Society; they also have an annual conference. The point being, this is a serious topic in the computer science and software engineering fields, with serious attention from both academics and practitioners, and not to be dismissed as trivial.
