Principles Trump Diagrams. Most of the problems in using the 1988 spiral model stemmed from users looking at the diagram and constructing processes that had nothing to do with the underlying concepts. This is true of many process models: people just adopt the diagram and neglect the principles that need to be respected.
Source: The Incremental Commitment Spiral Model, Barry Boehm,, Jo Ann Lane, Supannika Koolmanojwong & Richard Turner, Addison-Wesley