Rule #57: Don't take advantage of a drunk's lack of coherency
If he's at the point where you are serving him a water martini with olives and he cannot taste the difference between vodka and H2O, it does not mean you can still charge for a martini. Or if he has already paid his check but tries to pay again with cash, accepting the payment isn't exactly ethical, no matter how adamant he is about settling his bill...again.
Sometimes a bartender's job is to protect people from themselves. Excel at it.