Self-service software fail

Over the holidays, I noticed that the Home Depot near our house had installed a self-service propane bottle exchange system. It consists of a computer kiosk and a rack of cages for propane bottles.
I watched the demo:
Step #1: You swipe your credit card and make the purchase.
Step #2: The door to an empty cage pops open.
Step #3: You insert your empty bottle in the cage and close the door.
Step #4: The door to a cage containing a full bottle pops open.
Step #5: You take your full bottle and go on your merry way.
As soon as I saw it, I knew I had to try it. One of the big advantages is that you can replace your propane bottle even when the Home Depot store is closed. As a QA engineer, however, I knew better than to try it after hours.
So, last Sunday afternoon, I took my empty propane bottle over to the store and tried it out. It worked great until the end of step #3. After I deposited my empty bottle and closed the cage door, the kiosk screen thanked me for my transaction and did not continue with steps #4 and #5. No error messages. Nothing. I was left standing in the parking lot with a $19.97 debit and no propane.
I walked up to the service desk in the store, and explained what had happened. The service desk employee told me that it had been doing that. He added, in a matter of fact tone, “It’s computerized, you know” as if that explained everything. He grabbed his keys, took me out to a different rack of cages with padlocks on them, gave me a full bottle and said, “Good thing we kept these around.”
As a software developer, I’m proud to see that the products of my industry inspire such great confidence and excitement in people.