Again, treating a use case as a programmatic entity rather than an entry in a requirements document or a note on a 3x5 card. If you're planning on introducing Use Cases as a first-class entity in your designs, as e.g. the DCI folks tend to do, that's fine. But you might want to make that introduction before talking about use cases in that context. Otherwise folks like me who think about use cases in terms of bits of paper or entries in Pivotal Tracker may experience some cognitive dissonance.