It is amazing that a lot of projects get derailed because new details are found late in the project and some of these projects even become un-recoverable but sponsors still avoid discussing details during the initial phases of the project.
Understandably, at the early stages of the project, not much is known about the deliverables but sponsors and project managers must consciously allocate time early on to talk about potential risks and not put those discussions on the parking lot with the hope that eventually people lower than the managers will think about it.
Sponsors and Project Managers always say that "the devil is in the details" but let the devil remain uncontrolled. Control the devil - or at least identify the worst case scenarios - early on and the probability of project success increases.
The key is finding balance between defining deliverables and identifying critical risks that might derail the project.