[ubernostrum]

I will second this because I've seen it happen, both to me and to other people -- an environment that wasn't a good fit changes and suddenly someone excels, or someone excels and the environment changes and suddenly it's like they can't do anything right.

Also, having clear criteria set up in advance is essential. You should know before you go into the room (or pick up the phone, or whatever) not only what constitutes a pass versus a fail, but also what kinds of things in the interview signal more than just passing. And the pass/fail can't just be "regurgitated an algorithm we wanted them to regurgitate", because that tells you nothing about whether somebody's actually good to work with.