Sometimes it's not about the person, it's about the team and environment. I've been both on the competent and not-so-competent sides, and the environment I'm working in made a ton of difference (note: it is a feedback loop, so mistakes and lack of delivery cause your environment to get tougher, and vice-versa).

Perhaps the guy felt he was going to get a raw deal and so was spending the few weeks looking for another job? Who knows. Lots of potential variables that might have turned an otherwise stellar coder into a quivering mess. The plural of anecdote != data.