I had a pretty interesting experience with this. We were doing a bunch of interviews and decided to try out FizzBuzz. I wasn't doing this round of interviews,

I had a pretty interesting experience with this. We were doing a bunch of interviews and decided to try out FizzBuzz. I wasn't doing this round of interviews, but things ended up a bit confused with timings and I had a 30-40 minute very casual chat with each candidate prior to the actual interview. In each of those I ended up probing a bit on technical stuff and everyone seemed to have a decent base level of competence. But they all went to pieces over fizzbuzz in the interview. The one I actually saw, I had had a fairly in depth conversation about what it actually means to use "ref" and "val" in C#, but then watched the same guy freeze and barely scrape through fizz buzz. My conclusion is that some people just can't write code in that kind of environment. What makes it tricky is some people (myself included) find it easy or even fun - maybe I can write fizz buzz in a single LINQ statement...Those people can assume that everyone not like them can't code.