Be honest here. You are buying something - whether its a taco, bike, movie, whatever. You werent charged enough, 10 cents, $10, $20, whatever. Do you bring it up with the cashier? Or are you the type who gets a little giddy inside and not say a word? Is it being dishonest if you dont bring it up?
Happened to me this morning, bought a movie ($30) only charged $20, didnt say anything about it.
Another time, few years back I was exiting the mall and this dude walking ahead of me drops a $20 bill when reaching into his pant pocket. As I get close to the bill, I picked it up off the floor and put it in my pocket and scoot off to my car.