Errors can occur at all stages of the design process, but fundamental errors made at the conceptual design stage can be among the most elusive. Indeed, such errors tend to manifest themselves only when a prototype is tested, often with wholly unexpected or disastrous results. These, more than any other design errors, are invariably human errors, because a conceptual design comes only out of the uniquely human creative act of transforming some private concept from the designer's mind to some public concept that can be described to and modified by other humans. The creative act of conceptual design is the result of nonverbal thought (Ferguson, 1977, 1992), and as such often takes its initial public form as a sketch or drawing. If the concept is fundamentally flawed, this may be recognized intuitively at the instant the design is articulated in the mind or in words or drawings on paper, and so the concept can be rejected outright. But if the concept has a basic error that goes undetected, then the error tends to be all the more difficult to catch as the design progresses through evolutionary stages of modification and detailed design.

Whatever classification schemes might be employed or whatever distinctions might be drawn among identifiably different stages of the design process, the creative act of conceptual design is as old as civilization itself, for it can be argued that without such design there would be no civilization.