Connecticut routinely comes down on retailers that have scan discrepancies, although I think it was more common back when Sen. Blumenthal was still our Atty General. Much of the time, you get the item free if you catch it scanning wrong. My husband always makes the cashier at Stop & Shop not start ringing up our order until he can stand and watch how it scans. (In fairness, S&S is usually pretty good--Target routinely has issues.)
A few years back, Best Buy got into trouble for maintaining an in-store website that looked exactly like their regular website, but had in-store prices in place of the online prices on their regular website. Customers would do their comparison shopping online at home and decide Best Buy was the place to buy their TV or stove or whatever, but the price might be higher at the store, and when they asked the employees to match the online price, the in-store website showed the price was the same as the shelf tag, so unless the shoppers printed out (with that day's date) what they saw online, they got hosed. I'm sure BB got rid of that scammy system--they kind of have no choice since everyone has an smartphone now and can pull up the real site in a matter of seconds.
Sears, in my experience, has been very good at matching their online price on a couple of occasions. One time I wouldn't have known, except they didn't have a shelf tag for some small appliance or hardware item (I forget which it was) so I looked it up on the Sears app on my phone. When it rang up different I said something and pulled it up again on my phone and the clerk took care of it right there--didn't need to page a manager 15 times or anything.
Another thing I do with my phone: if I'm in the grocery store debating whether to by something like detergent or paper towels, I scan the barcode with the Walmart app on my phone and see what the price is. If it's a sale that's less than WM's price or if the price is within a few cents, I stock up and save myself a trip. If it's higher, I make the decision how much it's worth to me to save that trip/get it now vs. putting it off until I make a Walmart run. (During the month of December, the grocery store makes a LOT of margin on me so I can avoid going to Walmart or Target during Christmas season.)