In various places I've seen the claim that by designing a system up-front, you can significantly reduce development time. I.e. by spending an hour designing you can save a week coding. My problem is that I have never seen a situation where I found this to be true. So I want to know of any examples out there that people have where this would be true:

So:

1. What sort of problem arose during coding? (or was avoided?)
2. How could you have avoided (or did avoid) the problem by spending more time doing design?
3. Why was it (or would it have been) hard to fix the problem in the code?