I buy what I want too, but I do watch the items as they scan and now and then one will come up wrong. I stop the clerk and politely mention the error and ask if it can be varified. 99.9% of the time it can be varified an error on the store's behalf. One time, I read the sale wrong and when it come up to scan, I asked about it. The clerk politely showed me it was my error and asked if I wanted to leave it. I said no, I would take it anyway as I wanted the item, I just misread how the ad was worded.

 Most of the time I use the U-scan when I have no sale items. If I have sale items or items that have a security device on them, I will wait in line at the other check outs. Now days, there is enough check out options at most grocery stores, that I have issue stopping the clerk to varify an item price. In the U-scan, I make every effort to not hold things up.