I have heard of this but never used it before today, it took a bit of time but my $9 garbage bags were free. I was at Canadian Tire and the shelf price listed $7.99, but came up $8.99 at the cash. They went through the usual process of getting someone to check while I and everyone else in line waited patiently, the guy couldn't find the product so he came to get it and disappeared. The manager saw this so she came over and told the cashier to let me pay the lower price while she went to get my product back. In the mean time I waited around the next empty cash and saw the little Scanning Code of Practice sticker so I mentioned that to the manager, she had to read it, but obliged and gave me my money back. The details are here :http://www.cacds.com/onrecord/documents/EnglishCashierDocumentSeptember2007Final.pdf
The example shown is exactly what I saw at the cash. I hope I remember for next time, because this does happen often.