The waterfall model is argued by many to be a bad idea in practice. This is mainly because of their belief that it is impossible for any non-trivial project to get one phase of a software product's lifecycle perfected, before moving on to the next phases and learning from them.

For example, clients may not be aware of exactly what requirements they need before reviewing a working prototype and commenting on it; they may change their requirements constantly. Designers and programmers may have little control over this. If clients change their requirements after the design is finalized, the design must be modified to accommodate the new requirements. This effectively means invalidating a good deal of working hours, which means increased cost, especially if a large amount of the project's resources has already been invested in Big Design Up Front.