The traditional retrospective opens with "what went well." It feels good and teaches little. The debrief opens with a sharper question: what assumption proved false? Every plan rests on beliefs about the world, and the most valuable learning is finding which of those beliefs reality did not honor.
Plans are bets on assumptions
A plan is a set of assumptions wearing the costume of certainty. We assumed the dependency would be ready. We assumed the team had the authority to move. We assumed leadership attention would hold. When a quarter disappoints, the failure is usually not in the effort but in one of these hidden bets. Naming the false assumption is naming the real cause.
Surfacing beliefs makes them correctable
An assumption you never stated is one you cannot correct; it just quietly fails again next quarter. Writing down what you were counting on, then checking it against what happened, converts invisible beliefs into visible, revisable ones. That is how an organization stops repeating the same surprise.
Blameless by design
This question is structurally blameless. It asks what the plan believed, not who failed to deliver. That keeps the room honest, because people will name a false assumption long before they will name themselves, and the false assumption is the more useful thing to know anyway.
A false assumption usually points at a condition that failed. The next question locates it.
How this fits the system
The falsified assumption you name here is the first entry in the Mission Debrief.