Check the bill in supermarkets at check-out time, once in a while you can spot a mistake

Friends used to tell me all the time that they have cases where they find discrepancies in supermarket bills, and spend extra time just after billing to confirm that the items mentioned in the bill match with the items they had mentioned. This is more important in cases where there is a sale ongoing, or the store is crowded. However, I was a bit cynical about all this, except when it happened to me.
I was in a Big Bazaar outlet (a fairly popular supermarket type of store), and there was a good deal on some pieces of luggage, with a 40% discount (and we liked the bag, so it seemed like a fairly obvious purchase). Purchase done and the bag loaded in the trolley, we went for further purchases. After an hour of dawdling through the store, where we did more purchases, we finally were tired and ready for the check-out. It was near closing time, around 9:45 PM at night, and we finally made our way to the billing area.
It was late, and we were tired, and the billing was taking some time (one of the items we had purchased had a defective bar code and the stuff had to be sent back to the respective area to get a replacement). Finally the bill was ready, and we loaded our bagged items into the trolley, and were ready to rush to the car and get some rest. Just out of some habit, I looked at the bill, seemed fine, and then looked again. The amount of the bag did not seem right, and at a closer look, it was showing a 10% discount. A quick discussion with the wife, and it was clear that even though there were several items that had a 10% discount, we had selected the bag with the 40% discount. This was an amount where it seems that we had over-paid by more than Rs. 500, and needed to get the money back. We pointed this out to the manager over there, and he had to check with somebody from the luggage section (it turns out that the computer still had the discount as 10%, but the real amount was 40%). It took a fair amount of time to get the correction made, but it made it clear that we would need to re-check the bill every time.