It is absolutely true that a good percentage of my candidates fail due to stress. As an interviewer do everything you can to decrease the level of stress, but there's no way to eliminate it. I often note, in interview feedback, my subjective belief that the candidate's poor performance is likely to have been a fluke due to the particularities of interview conditions. But in addition to candidates who make errors under pressure, there are many candidates who are unable to perform very, very basic tasks. There are many candidates, for example, who mess up basic syntax; who adopt bizarre naming conventions which suggest a complete unfamiliarity with functioning codebases; who confidently propose solutions which make zero sense, and are unable to answer basic questions about those solutions, but simultaneously are unaware that they are not answering the questions you pose. Many of these candidates have sterling resumes. At a certain point, you realize that resumes are not a good predictor of performance.