  You know what my absolute favorite thing about work is? When someone brings in something that is broken, looking all mad and stressed, throw the thing on the counter and stare at me.
Like I know what the fuck is wrong with it. Why don't you tell me what it's doing? Usually I just say "Hi", and stare at them, until they realize that they might actually have to *gasp* talk to the salesperson. Stupid People. Another favorite situation of mine, is when someone is yelling and yelling and making a big huff, and I just turn to them and say, "Sir, we don't tolerate this kind of behavior in our store, I'm going to have to ask you to leave.
When you're ready to use your "inside voice", you're welcome to come back and if you can treat me like an adult, maybe I can do something for you". Usually, if someone has a problem, and they didn't purchase our extended warranty, there is nothing I can really do right at that moment. Sure, if they're good natured, or really nice and understanding about the situation, I am willing to bend the rules or pull some strings for them, but as soon as they call the phone a "piece of shit", use the word "fuck" in my store, are rude to me, or use threats to call the BBB or whatever, I pretty much just say there's nothing I can do.
It's for the same reason governments don't negotiate with terrorists, because if you let them get away with it once, they will try it again. So: The moral of the story. DON'T BE A FUCKING ASSHOLE! 
