This question tests your honesty. It is actually designed to eliminate as many candidates as possible. Professional HR managers don't ask this question because it's a bit insulting—they should never ask questions that they themselves would not want to answer.
Ever thought how an interviewer would react if you asked him about his weaknesses? But it's a kind of a common question, so be prepared for it.
TRAP: This question is tricky because it's not positive. The worst way you can answer is by playing up your strengths. You can't say that your weakness is that you work hard and care too much.
Although applicants believe self-promotion is the way to land a job, the evidence shows otherwise. Interviewers tend to give the highest ratings to applicants who are more concerned with being seen accurately than positively.
TRICK: The honest answer wins. Don't be arrogant. You can't use this question to prove how wonderful you are. However, don't pick out your worst trait, the one you would like to hide even from yourself. Choose a characteristic that bothers you and slightly conflicts with your skills and motivation.
Which of your traits is not deadly, just slightly uncomfortable?