Rapid prototyping should happen every time you need to add or change an abstraction. With the way many (most?) software projects work, with new feature requests

Rapid prototyping should happen every time you need to add or change an abstraction. With the way many (most?) software projects work, with new feature requests and requirements appearing along the way, this happens all the time. With new requirements, the old abstraction often doesn't fit 100% anymore. Applying a few band-aids to the abstraction is probably fine, but at some point you will need to reconsider the design, or you will end up with a design that does not match the problem it's solving, making the code difficult to follow and therefore error prone. In my experience, this need arises very frequently in complex systems with evolving requirements.