[skewart]

I've never asked anyone fizzbuzz, but I have seen people badly flub very easy algorithm questions that are basically at the same level.

In every case it was because they over-complicated things. They came up with elaborate logic that they didn't need and then got lost and didn't quite get to a working implementation. Picture, say, starting to implement fizzbuzz using two tries and an associative array.

I would guess that a lot of the people I've seen bomb these kind of interview questions are actually good, capable programmers, but they just got overwhelmed by interview nerves and got lost.