The Israelites complain that they are tired of manna. "That's all we ever get, day in, day out!" God rains so much quail on them that they get sick. Meanwhile, Miriam and Aaron slander Moses' wife, Tzippora, behind her back. When God confronts them, they say: "How come you only talk to Moses face to face? We're prophets, too, you know." God afflicts Miriam with leprosy. Why is she the only one who is punished? Well, say the rabbis, she started it!
We must be careful with our words. Complaining will often get you nothing but anger from the person you're complaining to. And if you talk about someone behind their back, they'll probably do the same to you. How do we keep the peace? We talk. We don't complain to -- or about -- each other.