The Vending Machine with Exact Change
A late-night vending machine accepts exact change, except when it develops its own definition of exact.
Photo: Unsplash.
The vending machine in the basement had displayed EXACT CHANGE ONLY for so long that nobody remembered it working any other way.
At midnight I wanted tea. The price was 25. I inserted a 10, another 10, and a 5. The machine counted 24.
I pressed return. It gave me two 10s and four 1s. This was mathematically consistent but operationally unhelpful.
The security guard watched from his desk. “It dislikes the new coins,” he said. He opened a drawer containing an alarming private economy: old coins, foreign coins, arcade tokens, and one metal washer.
We tried again. The machine accepted an old 10 as 10, a new 10 as 9, and the washer as 5. The tea dropped.
“You should report that,” I said.
“I did,” he replied. “They replaced the payment board. Now it accepts the washer.”
The next week, a contactless reader appeared. I tapped my card and the screen displayed APPROVED, but nothing fell. The guard opened his drawer and handed me the washer.
It still lives there on a piece of string, labeled EMERGENCY FIVE. The machine has been upgraded twice. The washer remains backward compatible.