This should go without saying, because most of us have been on the receiving end of poor customer support. However, it needs to be said because we constantly see (or commit) examples of poor customer service. For instance, there is this report from Kotaku:
Microsoft Creative Director 'Doesn't Get The Drama' Around Always-Online Consoles, Devices
If you aren't familiar with the drama, there's been rumors and "confirmed reports" that the next version of the XBox will require Internet connectivity to start games and apps. In other words, if you're not "always on," don't expect to use the next version of XBox very well. At this point there is nothing official from Microsoft about this. Yes, people were/are getting upset. Folks always do for rumors, even though they know it's rumors. What's the best way to handle this? It depends on the situation but typically, belittling your customers isn't the right approach. That's what happened in this situation.
The Musts Towards Business:
The Musts Towards Our Peers (who are often our direct customers):
Yes, there are exceptions. But these are good, general "musts" to follow. Note that I didn't say picking on your business folks or effectively shouting down your peers. That kind of interaction alienates people. Do it enough and you'll find yourself facing a "career altering event."