(We are currently running a promotion where customers can get 20% off their entire purchase with a coupon. On this day, I watch as a customer brings 5 items into the change room, picks up the first item, and brings it to the cash register, leaving the other four behind.)

Customer: “Just this, please.”

Me: “Sure. Was there a problem with the other four items?”

Customer: “No, no, I still want them! Leave them there for me. I’ll come back for them.”

Me: “You’ll…come back for them? I’m sorry, ma’am. I’m afraid we don’t reserve items.”

Customer: “I’ll only be a second!”

(I decide not to say anything and continue with the transaction.)

Customer: “Wait, I want to use my coupon! It’s a 20% discount!”

(I take her coupon and bring up her new total. She pays and leaves the store. However, she immediately turns on her heel and comes back in, heading towards the change rooms where she picks up the next item on the pile and brings it to me.)

Customer: “Hello, just this, please. And I have a coupon!” *takes out another coupon*

Me: “You know, ma’am, this isn’t necessary. 20% off the total price of all the items is exactly the same as the sum of 20% off each individual item. So you can just buy all your items in a single transaction.”

Customer: “Are you stupid? If I bought them in single transaction, I’d only be getting 20% off! But I have five coupons! 20 times 5 is 100! So now, I’m getting 100% off!”

Me: *speechless*

Customer: *winks* “Don’t worry, I won’t tell anyone how to cheat the system!”