The PM's job is to stop those doing the project from getting derailed. Literally manage the project. This means holding the stakeholder's feet to the fire. If the steak holder agrees to the terms they need to accept the repercussions of changing requirements, and their own misunderstanding.

Bad PMs don't hold the line. They don't signal early when bad things may be coming soon. They let all the shit derail productivity.

This is why systems like Agile were created. By making derailment a ceremony it became acceptable to remove the onus of the stakeholder to really make sure the project is ready and worth it.

edit: i should read over my dictated comments a little better