Rule #26: What to do when you give someone the wrong check.
Scenario #2: You drop the bill for the cognac drinkers and instead of paying for two expensive V.S.O.P.s the customers pays for two inexpensive beers. Did they realize what they were paying for? Maybe, maybe not. Unfortunately, once the bill has been paid, it's over. You cannot go back and ask them to cough up the difference. Depending on your bar's policy, you will either have to promo the mistake or pay for it out of your own pocket.