Yea a lot of people in this thread missed that point. It's still contrived problems, just testing comprehension rather than creation. But you can contrive much larger problems than you could ever ask a candidate to code, so you escape the ability for a candidate to practice your exact questions to some degree.

i kind of like the idea of taking them through your codebase, though. allowing the interviewer to concoct problems leads to something i'm very passionate about, and that's attempting to stifle the ego involved in the interview process. for whatever reason, intellectual prowess, and displaying it, exposes itself in interviews. the interviewer will ultimately end up trying to dominate you intellectually to satisfy their own ego. it's pretty pathetic, and rampant in whiteboarding.