Project planning is very important and critical to the success of a project. Project plans are absolutely living documents that will be updated throughout the course of the project. One way to mitigate the issues you bring up is to have a solid project risk management process. If you can identify these risks up front during planning (staff vacations, leaves, resignations etc.) then you can have a contingency plan in place. So in the case you presented while it is unfortunate that the staff members weren’t available if that was presented in the approved risk management plan, when you need to ask for more resources, management should be on board since they should have approved the risk management documents. It may not be possible to forsee every possible situation, but that is why project planning and risk management are carried out throughout the project.