When will I learn - part 2

Yet again, I didn't pay attention - LO was throwing his toys out of the stroller while in line at Walgreens so I was just trying to make sure we got out of there with everything. The cashier missed one of my coupons. Granted it's only $0.50 but every penny counts! That's the whole point of couponing. Grrrr... not worth it to go back and how could I prove it at this point? Frustrating.