"Oh no we've got a complaint! What should we do?" So many times I have worked with people who are terrified fo recieving complaints. But complaints are brilliant I encourage you to seek them out. There's nothing better than a complaining cusotmer as they give you a unique insight into how a customer sees your business and how you can improve. You should alyas encourage your customers or clients to feedback about your service. Give them as many ways to get in touch as they need - even REWARD your customers complaints.
The sad fact is not enough of our customers complain, they get pissed off, they walk away and they don't come back and what's worse you don't even know why. Now, the Holy Grail Customers, the Complainer - he'll tell you exactly why and therefore you should reward him for it - give him a free meal, a voucher, bottle of wine, bunch of flowers it doesn't matter - he deserves it. Thank him for bringing the complaint to your attention then DO SOMETHING ABOUT IT (this is bit pretty crucial :).
You've just got to love those complainers - they care enough to make your business better.
Disclaimer - some people are born complainers (you know who they are) do not reward them with flowers, wine or other gifts. Just scowl at them until they leave.