I clip coupons every week to try and save a bit. This week our local 'big' supermarket had a flyer in the paper with a great deal on paper towels. I don't usually go to that store, but I did to get the deal. So I get there and there is a sign on the door saying the ad was a mistake and the paper towels are twice as much. The manager says it was an advertising error and the law allows them to fix it. But how do I know they didn't do the mistake on purpose just to get people to the store but then they change it on the door? Is that legal.