Most of these you can deal with by sincerely acknowledging their complaints, taking it seriously, trying to do better, and trying to make sure that their experience, on balance, is at least as good as they were expecting. If that involves "comps" (free desert, a bottle of wine, etc) so be it. If this involves asking them to try their meal again next time on the house, and making sure the chef knows, the best waiter is serving them, and you personally go there to greet them, even better. Everyone makes mistakes, it takes an honorable person to own up to their mistakes and makes things right.