Same thing happened to Mrs. jmw. However, she got the cashier to take the $10 off manually. The theory she came up with is that she needed to use another club card due to a possible 1 coupon per x minute limit. I don't like the AGC anymore, so I doubt we will test this anytime soon.
I thought about that too...my wife was at an adjacent check stand with a coupon too(hers worked with a larger amount if stuff too) if the club card was the limiter I think either hers or my first wouldn't have worked....who knows.