I have worked as an assistant manager of a cafè, thus I have ample recollection of being the receiving end of various complaints. I agree that as long as the customers are paying for their food, they have the right to be served a meal that is reasonably satisfying.
However yes, especially during a busy lunch rush, some mishap or another can occur from time to time, and the source of the problem is usually not the fault of the server, to whom the customers usually bring the complaint upon. So it is important, as vyapti suggested, to maintain the civil, courteous manner with due respect and understanding for the hard work, without being nasty or whiny.
I know from my experience, if the complaint was justified and brought upon in a justifiable fashion, we were more than willing to correct the problem, but if they get unnecessarily mean and obnoxious, yes... it could get very tempting to serve them back something they deserved from our point of view...