A couple of years ago, I was on an interview panel for a position within my own team, at the request of my manager. The panel consisted of myself and two Business Unit managers. The candidates were both internal, one someone I'd worked with for 3 years and the other someone coming from another department. Both had decent reputations withing the company and similar skill sets.
At the conclusion of the secondary interviews, it was apparent that the gentleman from the other department had a lot more "polish" on presenting himself. We really found ourselves comparing candidate A (the person I already worked with) who could definitely grow into the role given 6 months or so, and be great at it, with candidate B from the other department, who really seemed to be able to come in and hit the ground running. In the end, candidate B received the position, with strong backing from myself. I even disregarded the misgivings of one of the other managers on the panel who'd worked with this gentleman, chalking it up to one bad experience.
Needless to say, I quickly saw that I'd made a mistake. Once in the position, candidate B has shown no aptitude for getting things done, no sense of viewing things through the eyes of the customer, and almost no technical ability whatsoever. He leaned heavily on the experience of another technical person on the team almost from day one; to the point that this person's word (even when he's blowing smoke) is treated as gospel, with no ear to any dissenting opinions.
To add insult to injury, my original manager has moved on to another position within the company, and this gentleman has been promoted into her position, effectively making me one of his direct reports. While I have no interest in a managerial position (I don't think there is anything wrong with a technical person staying a technical person), it is a constant reminder of how I failed to look past the "polish" and see the lack of substance. Not only did I do our team a disservice, but I believe I did a huge disservice to candidate A, who would have been great given some time to grow into it.