1. Thinking "I'll never get this job," and not bothering to apply. Sure, if it's a wild reach, put your time into other opportunities, but if you can make a reasonable case for yourself, don't write off an opening just because you assume others will be be better qualified than you. For some jobs, hiring managers may be willing to take a risk on a less experienced candidate who brings other strengths. I once hired a hyper-organized, detail-obsessed candidate who compulsively balanced her checkbook for a bookkeeping role even though she didn't have any experience, and she worked out great.
2. Being too quick to assume you're out of the running, so not following up. After a job interview, do you obsess over all the little mistakes you think you made, and beat yourself up for not giving different answers? Some people take this to such an extreme that they decide they obviously failed the interview, and so they don't bother to do any follow-up. Obsess if you must, but don't let that deter you from following up after the interview to reiterate your interest. Your self-assessment may not line up with theirs!
3. Downplaying your own fit for the position. I don't want an aggressive sales pitch, but I appreciate candidates who help me understand how their strengths and experience align with the job. If it's not obvious from your resume, be proactive about telling me the reasons you think you'd excel. This includes telling me about personal traits, not just work experience.
4. Not showing your enthusiasm for the job/how much you want it. Sometimes I interview a candidate and end up not sure if she's even particularly interested in the job. If you're excited about the job, say so! Hiring managers are human--we like it when we feel a real interest from you.
5. Getting so focused on whether the employer wants you that you forget to focus on whether you want the employer. This is the mistake that can cause you the most long-term pain. Remember, the goal isn't just to get a job offer--it's to get a job that you're going to thrive in. You don't want to spend the next year struggling to perform, or in a culture that makes you miserable, or with a boss who drives you to tears. So yes, work to impress--but also make sure that they impress you.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results and chief of staff of a successful lobbying organization, where she oversees day-to-day management of the staff as well as hiring, firing, and staff development.