I find, over and over, that the best way to diffuse unfair criticism or disgruntled people is through reflection/empathy. (even when sometimes it is a little forced). A simple "that is so frustrating, I'm sorry that happened to you. Let's make this right. What do you need from me?" said with concerned eyes and a caring smile can turn a bully into a bunny quicker than anything. And when I discipline myself to actually do it, it's also far less stressful.