Even despite the latest scanning technology at your local grocery store, cashier errors are still extremely common and more often than not they will be in the store's favor and not yours. This is because it is extremely easy for cashiers to inadvertently double scan an item or type in a wrong produce code for a similar item that unfortunately costs slightly more than the one you actually got.

Make a point of standing and watching the cashier she scans in your items. It is much easier to get your corrections done right at the time when she has your groceries sitting in front of her than once you have left the store or even after you have paid but haven't yet left the cashier. Once you have already paid and left the store, it is extremely difficult to prove that you only had four of an item that you are charged for six of. Likewise, if you purchase the most inexpensive apples and the cashier accidentally charged you for the organic ones, once you have left the store it is much more difficult to prove which apples you really had in the bag.

While some grocery stores have a "the customer is always right" policy others stand behind the policy of what you left the store is your problem.