Yes, and that works fine as long as your bag is of virtually negligible weight - like a plastic carrier bag. If it's a sturdy pannier, the checkout scales can no longer detect when you've put in a packet of razor blades, or a biro, or a toothbrush, or a birthday card, or a USB geek stick, or a box of matches, or a pair of shoelaces... or any one of a hundred other light things they sell.
It's the same at B&Q. If you buy something of reasonable weight and something very light, I've learned always to scan the light things first. Otherwise, the presence of the heavy thing on the scale stops it being able to detect the light thing and you get "PLEASE PLACE ITEM IN BAGGING AREA.... PLEASE PLACE ITEM IN BAGGING AREA... PLEASE..." Fortunately at my local B&Q, the lad who minds the self-checkouts is wise to this.
But it's an obvious bug that should have been fixed.