A different situation yesterday and I'm not sure if I should have said anything or not. The people in the checkout ahead of me at Acme supermarket bought a prime rib beef roast as well as over $200 in other items. There was a circular coupon that reduced the price from about $8.99 per pound to $4.77 lb. For some reason the coupon didn't scan, and the cashier called over a supervisor. The supervisor figured out the difference in price with a calculator and manually applied the discount. Then I noticed when the cashier hit the "total" button, the register applied the discount too. So they paid something like $10 for what should have been a $35 roast after discount. There were two roughly $25 discounts applied.
I didn't say anything and I don't think the customers noticed because their overall total was still over $200. I "think" if it happened to me I would have pointed it out to the cashier. I'd feel guilty about THAT much of a discrepancy in my favor.