A quarter of way through development of the project, you realize the main component has been left out of project specification. Business unit should be notified and should agree to changes in project specification.
When you (as Project Manager) describe this to a business unit manager in hallway, he publicly blames you for not doing job right. prepare down dialog which would happen at team meeting as you describe process you would use to deal with this situation. Make sure to prepare ideas for managing expectations and steps one can utilize to manage angry people.