We've all been tempted to complain about a difficult customer. Perhaps they were unreasonable, a bit light on brains, or just plain mean. Whatever the reason, we should remember that those of us who don't remember our history are doomed to repeat it.
Let's say you encounter a grumpy customer who just can't be satisfied. Afterwards, you complain about it to a co-worker. This conversation makes you feel better because it validates how smart you are, how dumb the customer was, and the unfairness of it all. Unfortunately, that conversation will stick with you the next time you work with a similar customer and you'll be unlikely to get a better result.
Here's another version of the same story. Let's say you encounter a grumpy customer who just can't be satisfied. Afterwards, you tell a co-worker about how frustrated you were by the situation. Together, you confirm that you did a good job but also brainstorm a few ways you can get better results in the future. The next time you work with a similar customer you get a better result because of what you learned from the last experience.