Andy you made me think. When DH and I go to the store, I usually watch the cashier ring up the items and he bags or bugs the bagger (we have a number of stores where you do your own bagging. Yay!
But, with the scanners reading the codes, so the cashier isn't punching in numbers, I haven't had a mistake made in ringing up stuff in quite a while. It isn't important to keep an eye on that any more. I should probably be at the bagging end.
Don't be so hasty in assuming that everything is okay if the cashier scans in your items. Here in California several years ago there was a big scandal about supermarkets not having their scanning system and prices posted in shelves in agreement. The shelf would be one price but the scanner would charge you a higher price! Sucker punch! They get lazy and you pay more than you accepted when you saw the price on the shelf. (Nevermind the old days when each item had a price tag...)
So many supermarkets adopted the policy that "if we charge you more than the posted price you will get the first item for fee." If you bought more than one you'd receive the remaining items at posted price. And when you caught them they'd always send somebody scurrying off to check the price, and if the shelf was lower they'd just remove the shelf price (presumably to fix things later).
This may be spooky-weird to you but in my occupation related to computer programming I have a strange ability to remember numbers. I could go shopping for perhaps 2 dozen items and then memorize the prices "on the fly" for all the items! This was one of my favorite shopping games, something I always relished when shopping, getting items for free!
I don't even know how I do it. I grab the item, I look at the price, I fixate... Then at the checkout stand I watch the register ring up, look at the prices, and for some bizarre reason I know when the price is wrong! From my experience I recall I'd get about 90% of the items I complained about for free! (This could happen every 2, 3 or 4 shopping trips.)
They must have gotten their act together because in the last 2-3 years I can't recall getting anything for free, or even asking for a price check. I think their self-imposed store policy ("free if we overcharge you") must have gotten them to finally be motivated enough to have the shelf prices agree with the scanner prices.
It was fun while it lasted!