Have you ever hired someone who was great in the interview and on paper but didn't work out? Any idea why?
Second, on to your question. I have hired quite a lot of people and occasionally they are amazing in interviews and on paper and don't work out in real world situations. Over time you get better at spotting these people but it is a risk you take. As with many things, interviewing is a skill and some people are quite good at it. Often times it is not a technical or job skill mismatch, it is a personality or a team fit match. That can take weeks or months to uncover. The best thing to do is to have a clear exit strategy for each person you bring in if they don't seem to work out.
Now I never hire a sales person based on his rolodex anymore. I find out if the sales person is truly passionate about the product.
Also, on that note, stay away from the sales guy who tells you that he will sell anything to anyone. My best sales hires in the past years have always been people who think that they are not sales people at all :)