Are you sure that these edge cases can be deferred without having to do a full re-design and re-write to accommodate them later? I.e., if you do an implementat

Are you sure that these edge cases can be deferred without having to do a full re-design and re-write to accommodate them later? I.e., if you do an implementation that doesn't include the edge cases, is this implementation something that can be used in the full solution, or are you 0% done with the 100% solution when you deliver the 80% solution? If the edge cases arise out of contradictory or unsatisfiable requirements, it might make more sense to refine the requirements than to defer the difficult part of the implementation.