Of course they backed up the server. Many customers look for a reason not to tip, it's like a game they play. Why do you assume the customer is right? They rarely are when they act like this. A tip is not an option in this case, it is part of the cost of the meal. IF something was wrong bring it up much earlier when there is opportunity to correct whatever it is, do not use it at the end to try to pocket 20% you owe. Is the customer sometimes right, absolutely, but many times they are just being cheap. Always remember, you can't spell customer without cuss.