In my opinion, a planning phase is very important, a successful plan has to be built up in a way that allows for dynamic changes. In other words, when the need for revisiting an earlier phase (ie. planning phase) a plan should have a feedback loop path that would allow the project manager to make adjustments and changes in order to satisfy the new requirement and meet the attained needs. This plan should be done in a certain way that it guarantees some sense of urgency and ensure that the project is following the timeline. I believe that going back to an earlier activity to fix some issues does not mean that the project failed at a certain point. Canceling some parts of a phase in a project and revisiting an earlier one should not be confused with failure.