Turning Limitations into Innovation [the broken link was removed] by Marissa Ann Mayer:
people working on it have spent so much time and are so personally invested that it’s too painful to walk away. They often know the project is misguided, yet they see the effort through to the painful, unsuccessful end. That’s why it’s important to discover failure fast and abandon it quickly. A limited investment makes it easier to walk away and move on to something else that has a better chance of success.