That happened to me the other day at JoAnn's. I bought a bolt of fabric and wanted to use the 40% off coupon; plus I had a package of calligraphy pens that I wanted to use the $5.00 off coupon on. I told the young lady at the register how I wanted to use the coupons and thought she had rang it up accordingly because they usually are very proficient. When she told me the price, I thought it was unusually high but went ahead and ran it through on my credit card. Just before I left the store I looked at my receipt, and sure enough it was just backwards.
I turned right around, and I'm afraid not thinking and sort of rudely, went straight back to the cashier in front of another customer, and pointed out the mistake. She profusely apologized and refunded the bolt of fabric on my credit card and rang it up the other way. After leaving the store, I thought about it, and I really didn't get the full amount off the calligraphy pens, but when all was said and done, I saved almost $30.00 on the bolt of fabric.
You can rest assured I will check my receipts much more carefully in the future before I leave the store.