Hiring processes are imperfect, sometimes all you have is a couple of hours to make a decision that could last years. I've been the hiring manager on 40+ hires in the last 5 years, sometimes I'm wrong, sometimes I knock the cover off the ball. What's at least as important is once you realize someone isn't going to work out or is affecting employees who have a good track record to get rid of the bad hire before they make good employees want to leave.