This is the best interview I've ever had because it was focused on real-world problems. Instead of designing an abstract pie-in-the-sky system, they asked about a real project I've worked on, zooming in on details just to bring up interesting areas of discussion. The coding exercise was actually a debugging & refactoring exercise, which is much more realistic than what most companies do. The interviewers were very open and upfront about the challenges they and the company face. Also, I can vouch firsthand that when you don't know an answer to a technical question, saying "I don't know, but my best guess is..." doesn't cost you the job.