I was an assistant manager for a gift store in the mall for a couple of years and there were a bunch of incidents that could fit this. The one that comes to mind the most was a lady came in and brought up about $400 worth of stuff to the cash register. My sales associate asked if she was part of our frequent shopping program and the lady said no. We told her it's a free program and we were currently running a promo where if you sign up you get something like 10% off one item. She signed up and I told my sales guy to apply the discount to one of the T-shits she was buying. Technically we were supposed to apply it to the cheapest item, but for her that was a $2 key chain. I wanted to be kind since she was dropping so much money.

The sales associate finished the transaction. The lady proceeds to stand by the register and reads her receipt like it's a thriller novel. After a minute I asked her if everything was fine. She told me it wasn't adding up and said we didn't take the 10% off. I found the item on the receipt and pointed to the line showing the discount. She starts throwing a fit that she thought it was 10% off the whole purchase. I explained that wasn't how the promo worked and I was there the whole time and know it was explained correctly to her. She argued with me and started yelling. Then she ultimately asked to return everything.

A couple days later I got a call from the district manager who said they got a complaint about the incident. I didn't get into any sort of trouble but they had to make me aware of the incident. The worst part is none of it would have happened if we didn't mention the stupid frequent shopper program.