One of the main considerations when doing resource planning is the external projects and operations going on that will complete for the same resources. Resources consist of both people and equipment.
At the start of the project, one needs to consider the state of the projects that the resources may be committed to. Will the resource actually be available when he is expected? Is the current project running late and unlikely to release the resource when expected? I have seen projects start off in the red when the planned resources were not available to start.
For resources not needed at the start of the project (for example test teams and environments), what other efforts planning to use them at the same time? I have seen train wrecks when multiple project try to go into test at the same time.
At the end of the project, one needs to be aware of other projects that may want to use your resources. Will they start to pull resources early? Does this project run the risk of delaying a high priority effort if the the project runs late? I have seen projects stretch out forever when resources get reassigned and the last few details do not get resolved.
Resource planning - getting the correct resources at the correct time and keeping them for the necessary length of time - is one of the keys to a successful project. Planning and execution can be spot on, but if the needed resources are not fully available, the project will run late if not fail outright.
