I was once called down to the electronics department because a customer was ringing through her order down there and had a coupon for cat food. The coupon was for one dollar off of any Iams cat food, 2KG bags or higher. She had a 1KG bag.
Me: "Yes, you'll just have to grab a bigger bag of cat food. This coupon isn't for this item."
Her: "Well, I've never had a problem before!"
Me: "You understand that it specifies which product to buy on the coupon?"
Her: "Well, normally the cashier doesn't look at that and they just take the money off!"
Me: "Clearly you have never tried that here, then. We manually type in each coupon; we can't scan them."