You're paying for four items at Trader Joe's, but notice your grocery checker has neglected to include one of the most costly. You:
Score some free blueberries, awesome!
Debate a second, then figure he's the one who didn't scan your blueberries, so it's not your fault.
Tell the checker, 'cause some cops are in line right behind you.
Tell the checker, 'cause you're an honest person.
Tell the checker, so as not to arouse the wrath of your chosen deity.