I know them, well do you really??
One of the things I see, and hear about all the time is how someone knows someone, who knows someone and how it either helps them get a job or stops them from getting a job. The interesting part is when the people who know people, knew them 1+ years ago, and yet still feel that what they know actually is still correct.
According to a study by Dr. Phillippa Lally of the University College of London, people have changes to the way the think, act, react, and process things at least every 4-6 months. That means if you knew someone say 1.5 years ago and back then they did not listen well, it is likely they listen very well now. Or If you knew someone 3+ years ago and back then they were a little hard to work with, it is very possible that they are now easy to work with.
In other words even though you may have worked with someone, or worked around someone, or even socialized with someone, if it has been more than 4-6 months you really cannot say you know them at all. The same would hold true if you are asking a third person about someone. If that person has not been around the person you are questioning about in the last 4-6 months they really cannot say they know. In fact if the person you are asking is someone you have not been around in the last 4-6 months you cannot say you know them well enough to have complete 100% trust in what they say.
In this age of social media were you can always find someone you can contact to ask about a perspective candidate, the reality is those opinions you get if are based on information 4-6 months or older are not 100% reliable, and in fact could be totally unreliable, and yet candidates are hired or not hire based on them.
Now I am not saying totally ignore them, what I am saying is use them like a piece to a puzzle, but talk to the candidate and address those issues and see if they are real, and are current. This way you are not giving up on what could be a great hire because of information that could be wrong.