A job candidate can be his own worst enemy. He can undermine his efforts without realizing it. This is common in interviewing. While you are trying to perform well, you inadvertently do things that prevent you from achieving your goal.
People compete with their own efforts to succeed in two main ways.
Preparedness First, a candidate's efforts can be undermined because he or she is unprepared. You know what you want, but you have not done what is necessary to achieve it. Perhaps you didn't know where to get the information you needed to prepare for your meeting. Perhaps you just didn't devote the time necessary to study and think. Or, you thought your knowledge about a subject was exemplary, but you were surprised to learn it wasn't.