It's wrong to rip people off.
Trent is working at a cash register and a customer gives him a fifty dollar bill, but they think they only gave him a twenty.
Trent wants to conclude the transaction.
Trent shows the customer that they gave him more than they thought and gives them the fair change amount.
Trent gives the customer change for only a twenty dollar bill and keeps the difference.
The customer is happy to have encountered an honest person and not to have spent too much.
The customer is upset when he later finds he's missing fifty dollars.