The Tale of the Damaged Debit Card

A young lady comes into the laundromat to pick up her drycleaning. I fetch it for her, and ring it up on the cash machine. She hands me a debit card to pay for it. I swipe the card in the machine. BEEP! "Card Error". Hmm. I try again. Same message. Once more. Same message.
She says, "Oh, I have problems with that sometimes because somebody put a piece of tape on the magnetic strip." I turn the card over and indeed there is a piece of tape there. The card is also scratched to hell. Anyways, I try it a few more times, then ask her if she has anything else she can pay with. She says no. She suggests that I try to put a bag over the card, which I have seen others do so it would probably work. However, the only bags we stock are made for holding about 40 pounds of laundry, so are very large indeed. Knowing that this woman expects me to try, I reluctantly pull out a bag and use the very corner of it to cover the card. Again I swipe it. It doesn't work. I swipe it backwards. No luck. I swipe it back and forth, back and forth madly in an attempt to show my frustration. Nothing.
I sigh, giving up. "This isn't going to work. Sorry."
"All right." She puts the defective card back into her wallet. "I'll try this one." She pulls out another, newer card completely identical to the first. Even the number is the same. I quickly realize that it is the replacement card for her damaged one. And guess what? It worked! Why she didn't give that one to me first I will never know. . .