This is what we call a " walk off' in our store, I wouldn't say it was a scam as such, it just means a person who was on that self scan has scanned items and "accidently "!!!! Forgot to pay, therefore the items will still be left on the screen, you then get a msg saying " would you like to continue" but yes if you don't notice there is still items on the screen you can end up paying for those items.I think one of the problems is, people just swipe their contactless card over quickly not bother about a receipt and dash off( some on purpose, some not) It has happened twice to me this morning at work, first episode the person had gone, second episode I managed to call them back.