It's probably happened to you.
You're at a department store, a grocery store, a restaurant -- and the cashier gives you too much change. Should you keep it?
Is it the store's misfortune and your lucky day?
Or should you do the right thing and return the money?
To find out what people do when they think no one's watching, "Primetime" set up hidden cameras in a New Jersey diner and gave the cashier a stack of extra $10 and $20 bills, to dispense along with the customers' change, as if by accident.
Over the course of two days, we watched as 46 different people were given too much change. What would they do?
Some, like Joseph Sergi -- who received an extra $10 -- noticed right away and returned the extra change right there at the cash register. Sergi said it was not just the right thing to do, he was also driven by his compassion for the cashier.
"I know from past experience the cashier always has to pay if she makes a mistake," he said.
For Jerry Frain, who received an extra $20, the motivation to return the money was more basic: "I'm an Irish-Catholic and my mother always told me if I stole, I'd go to hell You never forget those things."
Many other good samaritans spoke of the idea that "what goes around comes around," or "what you put out in the universe comes back to you."
They seemed to be evoking a popular concept from the 2002 movie "Pay it Forward." In the film, a social studies teacher gives his class an assignment to think of a way to change the world. One student comes up with the idea of "paying it forward" -- performing three acts of kindness with the condition that the recipients must, in turn, do the same for three more people.
"The notion that good deeds reciprocate one another is essential to human society," said Carrie Keating, a psychology professor at Colgate University. "It's what we count on to enable social interactions and social exchanges to work."
But while 18 of the 46 subjects returned the money right at the cash register, 26 people walked out of the restaurant with the money.
When "Primetime's" John Quinones caught up with college student Michael Giaimo outside of the diner, Giaimo said he didn't notice the extra change. "I guess I gotta start counting my change," he said. "Usually, I do check, but today I didn't."
However, Giaimo admitted that if he had noticed, he probably would have kept it anyway. "It's not my fault they gave me extra money," he said.
When given the opportunity to return the money after being informed of the "Primetime" experiment, Giaimo decided to keep it.
"I'm probably going to keep it for today," he said.
One man said he hadn't noticed the extra $20 bill with his change. But when "Primetime" producers watched the videotape, it appeared to show him actually counting the money as he walked down the stairs outside the diner.
Keating says that knowingly not doing the right thing can have a small, but significant, effect on society. She cites studies showing that, "once you start cheating on the little stuff, it's easier to cheat on the bigger stuff."
"Primetime" decided to up the ante -- having the cashier give out extra $100 bills, again as if by accident.
Two men returned the money at the cash register. But one, Arnaldo Mateo, walked out with the extra $100. "Primetime" spoke to him just before he got into his car, and he insisted he hadn't noticed it. Mateo returned the money promptly, as did 16 of the 25 people who left the restaurant with an extra bill.
Mateo said, "I mean, definitely, if I had noticed in there, I would have told her, 'Hey, I got an extra hundred.'"
Perhaps one of the other good samaritans said it best: "Karma's a bitch. So, you know, do unto others as they would do unto you."