The Job Interview

What not to say when a job interviewer asks, 'What's one big mistake you've made?'

It's a given that in any job interview, you have to know how to talk about your skills and your strengths. But many forget that you'll almost definitely have to talk about your weaknesses, too.

How do you respond to the common job interview prompt, "Tell me about a time you failed or made a mistake," without ruining your chances of getting a job offer?

The worst thing you can do is share a time you failed and in a way that reflects poorly on you, as revealed in a recent episode of CNBC's "The Job Interview," in which candidates interview for real jobs while being filmed.

Dave Tonnesen, co-founder of swimming school company Swim Kids, and his sister and assistant manager Krissy Bartlett, interviewed five of more than 100 qualified applicants for a manager position. In one interview, Bartlett asked the job applicant to describe a time he made a mistake at work.

The applicant described a time when he turned his back on a child in his swimming lesson class, and the kid, who couldn't yet swim, fell into the pool.

Instead of expressing his concern for the safety of the child or explaining how he learned from the experience, he said, "I thought I was about to lose my job. There's also liability, parents have the potential to sue you."

Tonnesen replied, "What I was looking for was your concern for the child."

Not only did the applicant miss a big opportunity impress the hiring managers, his response worried them.

"What we're looking for is when they make mistakes, how do they recover from it and move on," Tonnesen said after the interview had concluded.

That candidate did not get a job offer.

Talking about a mistake you made or a shortcoming you have can be tricky, but according to bestselling management author and CNBC contributor Suzy Welch, there's a smart way to respond.

The key, she says, is to talk about your failure or weakness in a way that shows how it affected you and spurred you to become a better worker because of it.

"Answers like 'I'm a perfectionist' or 'I'm a workaholic' are the red flags of phoniness," Welch says. "And no one likes to work with a phony."

Instead, be honest and transparent in your answer.

"Surprise them, and go there," she says, "with the caveat that obviously it's best if your weakness is not central to the very success of the business."

And most importantly, make sure that after you share an example of a mistake you made or a weakness you have, you show that you've grown by addressing it. Hiring managers are looking to hear how you improved from the experience.

"No matter what weakness you name, the important thing is what you say about what you've already done to fix it," Welch says, "and how you plan to continue that process in the new position."

CNBC's "The Job Interview" airs Wednesdays at 10 p.m.

Read more about how to ace important interview questions.

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Video by Brandon Ancil.