I didn't do this on purpose, but I exchanged a K-Cup coffee maker at costco. Their policy is to credit your credit card and then you repurchase the item instead of exchanging it.

Most merchants just automatically credit the card back, but at costco they will ask for the physical card. I usually use my hilton card for the first $40K of purchases to get diamond. I use my costco am ex card just as a membership card and when the clerk asked for the am ex card I used, for some reason I handed him my costco one. I didn't even realize I made the mistake until I downloaded all my activity from am ex later that week and saw what happened. I called Am Ex and they transferred the credit from Costco Am Ex to my hilton am ex so I kept the hilton points. I am not sure what will happen when it's time to issue my costco rebate check on that Am Ex since I didn't buy anything on this year and it will be a negative number.