In our business, we’ve seen and heard it all in interviews – even from some very smart people who might be expected to know better. Unfortunately, the competition among experienced and talented professionals is so intense, even one of the “7 Deadly Sins of Interviewing” can put an end to a promising opportunity:

• Lying, exaggerating, inflating – You can never recover from a lie. You tell the recruiter or the hiring manager that you’re still at your last job, but you’ve already quit or you were fired. What’s the harm, you tell yourself — after all, you did work there. Big mistake! You have to tell the truth—period. Someone will verify the facts of everything you say. If you try to rewrite history, it’s game over.

• Dressing like you’re “Dancing with the Stars” – Too tight, too clingy, too short, too low-cut, too casual – such wardrobe missteps can trip up both men and women. Not every job interview requires a suit, but you should wear professional attire that’s on par with or a step above the work environment. Do your homework ahead of time — ask the recruiter or one of your contacts about the typical attire. When in doubt, dress up. And try your outfit on ahead of time. The last thing you need the morning of your interview is to discover your clothes no longer fit you.

• Showing up late – Given the importance of an interview, you’d think people would be practically pacing the parking lot for the moment they can enter the building. But failing to plan for traffic, procrastinating, a wardrobe failure, and a dozen other things can get in the way. My advice is to arrive onsite or in the neighborhood an hour ahead of time. That will give you plenty of time to sit in your car or in a coffee shop to center yourself, do some deep breathing, and visualize a positive outcome.

• Not doing your homework – Seems obvious, but people miss this one all the time: They go into an interview without having done much prep. You need to arm yourself with knowledge of what the company really does. Read everything on the company website, especially press releases of recent announcements, product launches, quarterly earnings, etc. And make sure you do some research into who you are meeting, such as by checking the person’s LinkedIn profile. Who knows! You may even have something in common—whether a former employer or a shared interest—that will help you break the ice.

• Having no stories to tell – The interviewer just asked you to elaborate on the accomplishments listed on your resume. In your nervousness, your mind goes blank. You stumble along, but can’t come up with the specifics, and lose out on an opportunity to show how what you’ve done in the past matches the demands of the job. Prepare and rehearse brief stories (think one-minute sound bites) in advance. Each should have four parts: the challenge/opportunity faced, the actions taken, the results achieved, and the lessons learned. The best way to prepare is to record yourself and play it back to listen for how well and concisely you deliver the information.

• Talking too much or too little – The interviewer asks you a question that you’re not prepared to answer. In a panic, you do one of two things: Either you go on and on, hoping you’ll say something relevant, or you go silent. Having a mock interview with a coach, mentor, or friend allows you to practice, while giving you an opportunity to receive some feedback on your verbal and nonverbal communication. And while you’re at it, be sure to practice the questions you’re going to ask. Smart, strategic, and well thought-out questions will show the interview how you think, while helping turn the interview from a question-and-answer session into a conversation.

• Appearing desperate. Whether it’s what you say (“Will I hear from you soon?”) or how you say it (sitting on the edge of your chair), if you radiate desperation, red flags will be raised in the interviewer’s mind. Are you about to be fired? Why has no one else hired you? Once again, my advice is to rehearse. Get feedback on how confident you appear (making eye contact, smiling). Video record a mock interview and watch yourself to see if your inspired confidence or trigger concern.

If you think you can “wing it” or fail to prepare adequately, you’re at risk of committing at least one of the 7 Deadly Sins—and redeeming yourself will be very difficult. Avoiding that fate comes down to one word: Prepare!

A version of this article ran on Forbes.com.

Gary Burnison is the CEO of Korn Ferry and author of the upcoming book, "Lose the Resume, Land the Job.”