There is some kind of law here that the merchant has to display the product price (either on the package or on the shelf), and the price, if scanned, must match the displayed price. If this doesn't scan correctly, the customer is entitled to the product for free up to ten bucks.
So when I do my grocery shopping I pay attention to the prices of things (especially if they are on special, you'd be surprised how often the "specials" somehow don't get entered into the system) and try to watch as the cashier scans everything. Sometimes it's impossible to watch and unload the basket at the same time. Saturday, I couldn't do both and when I got home, I noticed that the swiss cheese fondue had rung up at $5.99 instead of $5.79. So I went back again on Sunday, bought the cheese again, pointed out the error, and got it for free. Tee hee.
Free cheese, you gotta love it.