Vending machine 1 has the drink I want. Unfortunately it does not seem to be taking dollar bills. I concoct a wily plan. I go to vending machine 2 which is taking dollar bills but does not have the drink I want. I put in my dollar then press the change return button and I am given 4 quarters. Score!
I go back to #1. I insert a quarter. I hear a sound in the coin return slot. My quarter has been returned with several more. I insert a quarter again. This time it sticks. I insert a second quarter. It is returned to me along with my previous quarter and several more. Further attempts see my quarters just returned to me. I leave without a drink, but with a pocket full of quarters. The vending machine basically paid me to not have a drink and leave it alone.