First, you need to be available for the customer. At least give them an opportunity to explain the problem. Though there may be nothing you can do for them (which is highly doubtful in the first place), you can at least hear them out so that, in the future, you can try to avoid the problem. Don’t blow them off as this manager did to me. I have never known any manager at any place I’ve worked (or eaten) to not immediately run to a customer’s table when there is a complaint. Because the last thing you want is for the customer to leave unhappy or, in our case, to never come back again.