In addition to Frank Patrick's excellent post of the Top 10 Sources of Project Failure, I have one more: No project manager.
In the past week, I've received inquiries from people, asking how they can successfully complete projects or programs without project or program managers. I tell them I don't know how to do that. I explain that if there's no one looking out for the whole project, or the series of projects in the case of a program manager, the effort will most likely fail. Then I hear questions such as, “If I have to choose between testing and project management, which one do I choose?” I'm often tempted to reply with “47” or “widget” — I want to respond to meaningless questions with meaningless answers.
Instead, I ask, “What value does this project have for you? Have you ranked this project among the other projects?” Once we start talking about relative value, the other person has a chance of understanding whether it's worth fully investing in the project or not.
If you have projects you're not willing to fully invest in, stop doing them. Cancel the project. But if you are willing to invest in the project, make sure you've got a project manager or program manager able to manage the project to completion. One person (not a committee) takes the responsibility for organizing, steering, and managing the project. If you're not willing to hire a project manager, don't do the project. Spend your money on something else.