The hardware design process must be flexible. Not all requirements are always known from day zero. The changes are necessary, and I always keep in mind plans B or even C.
Variability
The project plan is being modified every week. Being flexible and agile is the most important part of project planning. Frankly, the project plan is just a wish at the beginning. More than one project variant must be considered during the entire development process.
Improvisation
Another crucial property of a good hardware designer is improvisation. Even the best agile development project needs a non-zero portion of improvisation. To be effective and agile, one must always be ready for improvisation.
Both variability and improvisation lead to a successful development project. None of them can be omitted.