Several of my colleagues are concerned by their “back-door” references, or unintentional references. They've gone on interviews, felt as if they'd aced the interview, were told “we just need to check references” and boom, they are out of contention for the job.
One colleague was told, “Oh those people you worked with > 10 years ago, that reference wasn't a good one.” When the colleague protested, “I know. That person and I had trouble resolving our conflicts, which I explained. That's why I didn't provide that person as a reference.” The candidate could not persuade the hiring manager to look at a more recent reference or try the candidate temp to perm, or any other alternative.
If you're providing a back door reference, please be careful. What you knew about the candidate several years ago may no longer be true.
Candidates, if you feel as if your back door references hurt your chances of landing a job, first touch base with your references. Make sure you know what your references are saying about you. Make sure you've chosen enough references. If that doesn't work, be honest with your hiring managers. “When you contact my references, you might find some back door or unintentional references at the same company. If you check references with them, you may not hear nice things about my work. Let me explain why.”
Back door references can hurt candidates. Before you, as a hiring manager, listen to them, learn what you are listening for. Someone who wants to prevent someone from finding a new job? Or honest information? As a candidate, make sure you don't take your references for granted.