two things you might hear in this situation are "be entreprenurial with edge cases" and "if you write robust edge-case-ignorant code, the modifications later should be easy". It's not immediately obvious how to respond in these situations.

trying to work through things on your own before coding, the fifteen-minute clarification process can start looking like a few days. and there's also no guarantee that you'll think of everything.

as for writing code that doesn't break when you work in the edge case you hadn't thought of, well, i guess that comes with experience.