I learned the Classic Development Cycle quite differently from most people (it seems). My high school Comp Sci instructor (1979) presented this basic software development flow and then immediately used it to introduce concepts of iteration, recursion, and feedback (on the model itself). I never felt that the Classic Development Cycle approach was flawed, just most people's interpretation and implementation of it (as a one-way monolithic process). To my mind, modern methodologies like XP seem like natural extensions or revisions of the classic model (as I learned it), not radical departures.