When projects derail

What happens when a project derails?
1)Why should a project manager develop a written project plan? If a project is being performed under agile approaches, does it still need a plan? What are the drawbacks to having a written project plan?
2)What happens when a project derails?
3)What are "lessons learned meetings" and why are they important to a project team? When do you think these meetings should occur? Who should run the meeting, and why?