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After completing a design session, in Process Modelling or Software Design format, you may feel like you finally cracked the puzzle.
Your flow is behaving well, and everybody seems to agree on the principle. But the feeling of victory may be premature. You may have reached an agreement in principle while still leaving some invisible gaps.
You may want to make your flow more robust by extracting the desired behaviour of your system into a set of acceptance tests.
They don't have to be formal; you can still use low-fi supports like index cards, but they must be unambiguous: when these actions and events occur, we expect the system's state to be this.
The "Given ... when ... then" format from Behaviour-Driven Development is a great tool for injecting precision into your narrative without displacing the conversation.
This approach works very well together with Names and Numbers and Raise the Bar. Corner cases may be a perfect background for testing the real expectations from every perspective.