Shopping with computers

In Sainsbury's with five items to buy. I know, to save time I'll go and use the new self-service checkout machines. Item 1: Swipe - 'Please place item in bagging area'. Item 2: Swipe - 'Please place item in bagging area'. Fine. Right, wait a minute, before things get out of hand let me just get my re-usable bag out (earning green brownie points, or should that be greenie points?) for my five items. I put said bag in bagging area. 'Unexpected item in bagging area'. Unexpected? Unexpected I ask? How can a bag be unexpected in a bagging area? Someone hasn't thought this through...