Lack of a clear, shared, well-understood objective - if you go around everyone in the team (i.e. not the managers) and ask them what they think the next milestone is, and what it's supposed to achieve / represent, and they don't all say the same thing, then you're in trouble. Projects often unravel towards the end - processes fall down, communication ceases, and people start working blindly through a list of never-ending tasks, with no clear idea of when they're complete.