|Your manager should be spending more time reading management books and less time reading the riot act. Rather than playing the come-down-very-hard card, your manager should have played the investigation card. He or she should have discussed the situation with you and followed up with the customer as well. There are obviously two sides to the situation, and before taking any kind of disciplinary action, both sides should have been fully explored.|
On the one hand, it is possible that your admittedly unfriendly reaction to the customer inflamed the situation. At the same time, it is a complete myth to think that the customer is always right. There are countless situations in which customer behavior is obnoxious, destructive, and even illegal.
Since you have had a successful work record with the company, and it does not sound like you are frequently in a battle mode with the customers, you can look at this event as a learning experience. Hopefully you learned that lowering yourself to the level of an abusive customer does not do anyone any good. There's also something to be learned about communication, support, respect, and trust in your company. You may be in a sales organization, but management may have forgotten the importance of treating employees like customers.