Target does it all the time. Right at the checkout...no need to even take it to the customer service desk. I found this out by accident - I had two coupons for $5.00 off a purchase of $50.00 or more, and gave her the wrong one. Before I could even give her the correct one, she said not to worry about it, pushed a few buttons, and overrode the error. OP - I never expected the lady at Target to override anything for me, either It never hurts to ask!