My pneumatic tube story about things that can go wrong with them: A customer pulls up to the drive-thru, puts their money and deposit slip in the carrier and sends it off into the building. The teller receives it but it's empty and tells them "I'm sorry, but you need to include your paperwork" and sends the carrier back. The customer holds up the carrier so the teller can see that there's paper & money in there and sends it off again. The teller holds up the carrier they received at the window, and it's empty!
What had happened was a previous customer had driven off with the carrier, and the teller put their spare into the tube. Then the customer who had absconded with the carrier realized what they had done, and returned to the drive-thru lane - putting the carrier back in and leaving without saying anything. Now there were two carriers in the tube. So when the next customer arrived, the carrier that arrived at the teller was the spare, the one that didn't have their documents in it.
What had happened was a previous customer had driven off with the carrier, and the teller put their spare into the tube. Then the customer who had absconded with the carrier realized what they had done, and returned to the drive-thru lane - putting the carrier back in and leaving without saying anything. Now there were two carriers in the tube. So when the next customer arrived, the carrier that arrived at the teller was the spare, the one that didn't have their documents in it.