Yes, the customer is always right. Even when they're wrong. I handled a complaint today. It was a minor one, but a lady asked for a manager and I happened to walk past at that moment. Lucky me! You never know what to expect when someone asks for a manager.
We were standing next to an empty conference room, so I invited her in. My first rule of dealing with this kind of thing is to try to go somewhere that is private. That way other patrons don't have to witness it when/if it gets heated. That's just embarrassing! It also shows the patron that you care enough to give them some privacy. We pulled up chairs and I introduced myself as the Adult Services Coordinator (I don't think that impressed her. What the heck is an ASC, after all?)
My second rule is to ask them to explain what happened. Sometimes this takes a while. They start talking and they keep talking. There's no point in interrupting to ask questions. They have something to say, and by gosh they're going to say it. Make eye contact, nod thoughtfully, and just let them get it all out. Eventually they will stop talking.
That brings me to rules #3 and #4: be sincere and do not be defensive. Remember that the patron is always right! Sometimes, they are clearly wrong, but if they bothered to ask for a manager to complain, they get to be right. Apologize if need be. If the patron is clearly wrong, you just say "I'm so sorry you had a bad experience at the library." Don't offer excuses. Ask what they would like to see happen. Then compromise if you need to. No, the staff person won't necessarily be fired, but the staff person will be met with and perhaps re-trained. A policy won't necessarily be re-written, deleted, or created, but it will be discussed in a manager's meeting. Offer to get back to them after it has been discussed with the Powers That Be.
In my experience, after you listen carefully and sympathize, the complainer is often done. They just wanted to be heard by someone in a position of power. When the manager gets defensive ("My staff would never do that!" or "That's not our policy!"), it fuels the complainer even further. The injustice of it all! They will NOT be treated that way! Your staff did TOO do that! It may not be your policy, but IT HAPPENED!
The most difficult part, to me, is suggesting a compromise. It's not that I don't want to - heck, I don't care, just give 'em what they want! It's that you don't want to promise something you can't deliver and you don't want to pass the buck to your boss, either. As a middle-manager, I need to just be able to deal with complaints. I can't pass simple complaints, like the one I took today, up the chain. I have to come up with something reassuring, sensitive, and sympathetic to say that doesn't break too many major policies. I suggested that the staff member be asked about it, that the managers discuss it in our meeting, and that I call her back after those two things have happened. This promised follow-through, but gives me a chance to see what the other managers think - and get the staff person's side of the story.
When the patron is clearly wrong, it is difficult to stay neutral. You want to defend your staff and your library! But, the customer is always right. Always. Even when they're wrong.