Sometimes a customer makes a downright irrational complaint. I don't really understand what they're complaining about or how to resolve the issue.

So I ask them to fill out a simple form: What happened? What did we do wrong? What is your desired resolution?

Often by the second question, they begin to realize we did exactly what they wanted (and no I don't mean we did it in a smart aleck way) and that maybe they're being irrational. By the third question, they get a perspective check: "how would I do this?"

Or they don't... and then explain in detail what they didn't like. We then address the complaint to the satisfaction of the customer.

I only use this in specific situations, but it's very effective.