Even if your job interview goes brilliantly, and you bring everything to the table that the hiring manager is looking for, one mediocre reference can cost you an offer. That's regrettable -- and maybe even a little unfair -- but it's also avoidable.
Employers won't generally contact your current and prior colleagues at random. Sure, when it comes to some high-level jobs, the person doing the hiring may know someone who has worked with a candidate, and ask that person for their view. In most cases, however, the process is confidential, and the only references that will be called are the ones that the applicant provides.
Because of that, you have the ability to somewhat control what employers hear about you. So make sure the reference list you hand over contains only people who will speak highly of you.
You control the narrative
Many years ago, I ran a large toy store. At one point, a former employee called and asked if I would give her a reference. I told her the truth: I said that I liked her, found her work with customers was excellent, and that she had generally done a good job. I also made it clear that I would answer the questions asked -- so if the subject came up, I would have to mention her spotty attendance record and general lack of reliability.
Somehow, she still used me as a reference, and the hiring manager who called did not ask me a question that prompted me to reveal her weaknesses. That was a lucky result for my former employee, but she walked a razor's edge. The wrong question -- even one as simple as "Is there anything else we should know about this person?" -- would likely have resulted in a less positive outcome.
Before you list someone as a reference, ask them if them if they're on board with that. Give everyone a painless opportunity to opt out. Ask them directly: "Are you comfortable recommending me to potential future employers" and "Is there anything negative you might say about me?"
Those might not always be the most pleasant topics to broach, but it's better to hear something bad about yourself privately than to have your flaws exposed to a recruiter or hiring manager. Hopefully, even if one or more of the people who would have been among your first choices reveal that they might say things you'd rather not have said, you'll be able to find others more willing to sing your praises.
Cultivate your references
Lining up references is a task best worked on at a time when you're not actively looking for a job. Ideally, you'll want people from different categories -- bosses, peers, and even people who reported to you. And it goes without saying that the best way to ensure you'll have good references when you need some is to do your job well enough that you deserve them.
When you start looking for a new position, call those potential references. Crafting your list will become easier the further into your career you go, as you'll have worked with more people, but those who are early in their careers do have options. If you've only held one professional job, it's perfectly OK to list part-time employers, or even college professors -- or high school teachers if you did not go to college.
Look critically at your reference list to see where it might have holes. If you've never had a boss who will speak highly of you, work to change that -- and develop alternatives. Your list may not be perfect, and if so, that will take some time to fix. But it should be the best possible list for you at the moment when a potential employer picks up the phone.