If the chain is broken in just one place, the bicycle cannot move. PM has to take such cases into consideration when preparing the project schedule. One of the most important things when creating development plan is a good cooperation with game designer. Concrete, detailed design document is a key. When you know exactly what to do you can focus on how to do it and how to make it best.
Many people tend to improve their work again and again.
Especially programmers love optimizing their code and not rarely spend an extra month to achive one frame per second more. In other way the project will be late. And last but not least - the leadership. At the beginning the whole team works with enthusiasm and passion. It's up to the Project Manager to keep this state permanent and to direct the team to the right way.