Many of these stories are great! Thanks for sharing. I've had some experiences too, but nothing as good as some of yours.
During the checkout at a grocery store, I had to ask about the discount program when they mention it. "I don't know what that is," I said. The cashier replied to me, "It's our online discount program, but honestly, the discounts aren't very good and I don't bother with it." Oops, busted!
Another grocery shop, the cashier handed me my merchandise and said, "Thanks for shopping at Walmart!" (I was NOT in a Walmart.) She then immediately gasped and put her hands over her mouth. Eh, it amused me.