Anyone who has honestly reflected on what they know about hiring, will tell you that no matter how locked-down you think you have it, you dont. There is still way too much left to chance and way too much that you just dont know. To avoid this, companies that care about preserving their culture will sometimes adopt a "default deny" approach. It's ok to miss a potentially good hire rather than to take on a bad one. This isn't silly geek risk aversion.. It's because one bad hire can do amazing damage to a culture (an area bad hires can be amazingly productive in).
We have been hiring and interviewing people for about 7 years and have learned many lessons along the way but without fail, the one that works best for me, is the T-Shirt test.
The T-Shirt test is simply to ask yourself: "how will i feel standing at a conference, with this guy next to me wearing my company T-Shirt". If you don't like the thought, you shouldn't make the hire.
Now this doesn't translate to only hiring good looking people (but if whats important to you when standing up at a conference is the looks of the person next to you - you probably dont read this blog). The immediate question that comes up is "what if the guy is a real rockstar - but isnt presentable?". In truth, im happy with that, and far prefer it to a very presentable GQ model with a brain of a pea.
The T-Shirt question is a good evaluation of how you feel about the candidate and personally i can tell you that everytime i have ignored it, i have kicked myself a little later.
Interestingly enough, this doesn't have to be just for tech-geeks. If you are sitting at a management level or at a board level, the same question applies. I.e. How do i feel about standing next to this person at X, and how do i feel about him representing me / my brand. If your answer isn't positive, either he shouldn't be there, or you shouldn't..