Colleagues were discussing a vending machine gone wild the other day. It was giving out incorrect change—but happily in the favor of customer.

Put in a dollar and a quarter for a sweet roll marked $1.25 and it would spit out the product and return two quarters. Put in four quarters for a 95-cent candy bar and it would dispense the item and kick back a quarter and a dime.

In short, the machine was having an unauthorized sale, and customers were taking advantage of it. In economic terms, deep discounts were fueling heavy demand.

“But isn’t that stealing?” a ...