We've all had it happen to us. We were able to get a signed agreement in hand, identifying agreed upon scope of work. Everything for a fleeting moment is right in the world. Then it happens. That one stakeholder (you know who they are) comes to your desk and asks. "Can we add this one little tiny feature?" or "Can we make this one tiny little change?" Are you kidding me? This reminds me of when my son asks if he can have dessert when he hasn't eaten his dinner. Though you can't be as abrupt with a stakeholder like you can with a 4-year-old, the answer should still be the same. No.
Though you should not be an obstructionist, we could all learn a little from Dr. Cox in this case. His (command) mitigated speech is all he needed. In the real world, stride to be a win-win negotiator and be aware of the mitigated speech being used to conduct your negotiations.