The waterfall model 
for software development works poorly as a conceptual metaphor .
Royce 1970 did not name the waterfall metaphor, but he introduced the diagram that inspired the name:
Double-click to enlarge
It’s easy to see why the process came to be called “waterfall.” It’s nice to envision water flowing over that series of ledges.
But that's a a horrible mistake. Water flows easily and naturally downhill. Software development isn't like that.
This is a better picture, more faithful to reality.
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Each step is *work*. Fish Ladder Analogy
Beyond implying software development is "as easy as falling off a log," there's also the Code is Down problem.