Nobody has ever run a complete non-trivial project as a pure waterfall. The paper often cited as the originator of waterfall by Royce first of all doesn't use the term and second makes it clear that a linear progression through steps is not desirable or even practical. You always need the ability to go back and revise what was produced earlier in the project. Waterfall as generally perceived is nothing more than a straw man used by agile methodologists to frighten those who don't know any better.