It turns out that it's possible for your references to diminish your chances of getting the job without even saying a word. Here's a recent experience I had when checking the references of two candidates I was torn between.
Candidate no. 1 gave me the phone numbers of three former managers. I reached each of them easily and their references were strong.
Candidate no. 2 also gave me a list of references. One featured a phone number which, when I called it, turned out to be disconnected. Another featured a number that was answered by someone else, not even at the same company. I had to contact the candidate several times to get correct numbers. This raised red flags about the candidate's attention to detail and whether she doesn't tend to check things out before handing over potentially wrong information. In the position she was applying for, that could be disastrous.
The lesson? Call your references yourself ahead of time. Not only is it smart to give your references a heads-up that they may be called by a prospective employer, but if you haven't talked with them in so long that their contact information may have changed, you want to find that out and track down the new details. You do not want to give the reference-checker bad or outdated information.
There are other ways candidates sometimes mess up with their references, such as:
Not staying in touch. If your old boss, who will give you a glowing reference, leaves her company, you need to be sure you'll have her new contact information. I'm always surprised by the number of people who have to scramble to figure out how I can get in touch with their former managers.
Not offering up managers as references. If all your references turn out to be peers or people who didn't directly manage you, I'm going to wonder what you're worried your managers will say. (And I'll ask to talk to them anyway.)
Using friends as references. If the reference-checker discovers that the reference is a personal friend, it undermines that person's credibility. I was once talking to a reference who mentioned that he used to date the candidate--this is not a good idea.
Alison Green is the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results. She is chief of staff for the Marijuana Policy Project, a nonprofit lobbying organization, where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.