Here I have to watch scanner as well, but as we don't coupons I don't have that problem. But at one supermarket chain they don't even let you see what they are ringing up so you need to be extra careful when shopping there.
As I do check all dockets before I leave the area in all supermarkets, ithat I can be seen from the checkouts by the staff which can be advantage at times too!
Last time I went there, I bought Fruit & Nut Mix which was on a Thursday & sales alway start on Monday's here. So on checking the docket I was charged full price and not special price in the catalogue for that week. So I end up getting it for Free.

They were not happy that I did pick it up I would think, so how many others had gone through paying full price.
