As a bar manager, here is what we do in these type of situations. Its not very often that somebody disputes the tab, and if they seem pretty sober, I will often comp the difference knowing what our cost is. I never let a staff drink, so if its a staff versus patron situation, I will side with the staff nine times out of ten. As for a card being valid, almost always a card will go through. You might grab a few cards a year that are not valid, but that is the risk you take. If the bill is high, I will often run it for a certain amount during the course of the night, just to make sure it will go through. As for tabs being left, if its a small amount, I will let it slide. If its a larger amount, I always add 10% to any tab left to make it fair to the bartender. Very often, people come in and want to leave a tip, but once its run and processed, we can't run a tip unless we run the card again since it was settled the night before.