This is not a dig against cashiers. In this computerized aged, most cash registers have scanners. These little marvels can be inaccurate, They can also be mis-programmed. The sooner you can catch a mistake, the faster it can be corrected.
This will save you both money, and time.
This will also mean that you should not be doing these things:
talking on your cell phone, daydreaming, or wandering off to look at something you're not even going to buy.
It's your money, and if you want to control how it's used, you need to pay attention to it.