1. You have multiple short-term jobs. If you have a history of quickly moving from one job to the next without staying very long, employers will wonder whether you get bored easily, or can't keep a job, or don't know how to identify the right fit for yourself. If you do have good reasons for the job changes (such as a spouse in the military), make sure to fill in your interviewer quickly so she doesn't draw the wrong conclusions.
2. You quit your last job with nothing else lined up. Since most people line up a new job before quitting an old one, employers raise their eyebrows if you left without something new waiting. They wonder what the real story is: Did you blow up one day and walk off the job in a fit of anger? Do you get upset at work and make impulsive and rash decisions? Were you actually fired but trying to claim you left on your own?
3. You were laid off from your last job. While plenty of layoffs are about company cutbacks or restructuring, employers know that companies sometimes use them as an opportunity to get rid of lower performers. To combat this question, be sure to mention if your whole team or division was let go. If you were the only one laid off, that raises more questions than if you were part of a group that was laid off.
4. You've been unemployed for a while. Even in this economy, some hiring managers look at long-term unemployed candidates and wonder if there's a reason that other employers haven't hired them. Fortunately, many employers do understand that it can take time for even good candidates to find work in this market—but it's important to show that you've been spending your time volunteering, building your skills, or something other than a year-long job search.
5. You have large gaps between jobs. When employers see gaps of unemployment, they wonder what happened during that time. Did you leave the previous job with nothing lined up, and if so, why? (See No. 2) Were you working somewhere that you've deliberately left off your resume, and if so, what are you hiding? Gaps raise questions that you don't want on a hiring manager's mind.
6. None of your past managers are on your reference list. If you only offer peers as references, or other people who didn't directly supervise your work, hiring managers are going to wonder why. Managers are usually best able to speak to the quality of your work and your strengths and weaknesses, and steering reference-checkers away from those conversations can be a red flag. Plus, employers will usually ask to be put in touch with your past managers anyway.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.