This is one of my favorites sayings, because it reminds me that however careful you are at hiring, you are going to make mistakes. Hiring is definitely the hardest thing you have to do as a manager. People try to deal with the difficulty by using grueling interview processes, and requiring unanimity before hiring anyone. In my opinion, too elaborate an interview process is counterproductive, because it tends to just reinforce prejudices. That leaves you with too homogeneous a team in approach and outlook. Take chances on people with different backgrounds, and the diversity of your team will make it stronger. But, you will still screw it up.