Do you let the cashier know when you've been undercharged?

I generally buy the same thing at my favorite coffee place so I know how much it should cost. When they tell me a lower price I always point it out. But it's not so much because of honesty as concern that I get the drink I wanted. Maybe they misunderstood my order and that's why the price is different. Besides they take good care of me so I feel some obligation to return the favor.

I had to correct a particularly clueless employee at this same place after she gave me back change that was more than I'd given her in the first place. Actually I gave it back to another employee because I thought she needed to know the other employee needed some, uh, help.