In my experience, one generally starts following the process ideally: Initiating Processes, Planning Processes, Executing Processes, Closing Processing. While trying to follow the process, problems usually occur that may require you to revisit a previous step. For example, during the executing process you might run into problems that require you to go back to the planning process and review and reassess your plan to mitigate risk and problems. I don’t really see this as a failure but more of a learning experience. One can learn about problems and risk from past similar jobs that have documented lessons learned but sometimes, the problem is completely new. When the problem is completely new this is the nature of the work. You are going to try things and usually, you will learn about problems/risks during the executing process and have to reassess your plan to further mitigate other problems and risks.