There is a reason why I'm always saying "check your receipt"! I've had 3 problems at 3 different stores in the past couple weeks. If I hadn't noticed the problem, I would have wasted over $15! Two of the issues I caught while checking out and one of them once I got to my car. Remember, to always be kind to the cashier while pointing out a problem. It is normally the computer's fault and not the cashier anyway. If you are nice to them, they are usually nice to you :)
Mistakes happen, just make sure that you don't have to pay for them!