I am a big fan of agile methodologies at all – if done right. Therefore, in my lectures, I teach about the big misunderstandings of the waterfall model, which never was told as such by Winston W. Royce in his paper “Managing the Development of Large Software Systems”. Actually, in his original paper, he described the error-prone approach he observed at this time.
“I believe in this concept, but the implementation described above is risky and invites failure”
Eventually, Bell & Thayer called this process Waterfall the first time in their paper ”Software requirements: Are they really a problem?”. 10 years later it became a standard in the Department of Defense Standard 2167A (DOD-STD-2167A).
When reading Royce’s paper carefully you will find a lot of recommendations pointing to nowadays agile practices such as iterative development, talking to the customer, performing reviews and even DevOps approaches as the fact software should be run by those who write it. All these things mentioned by Royce are never mentioned when talking about the waterfall.
But then again, when talking about agile you probably will realize, that it is nothing else than “Waterfall” just in small sizes as illustrated by @JishuaKerievsky:
Royce’s original paper: http://www-scf.usc.edu/~csci201/lectures/Lecture11/royce1970.pdf
Paper by Bell and Thayer: https://pdfs.semanticscholar.org/a50d/8f564d063e526e94114875220440f64d8666.pdf