While in a fast food restaurant the other day to get my iced tea after exercising the customer in front of me had a problem w/the cashier.  The customer had ordered and misunderstood the total bill; $7.74 is what he thought he heard, but it was actually it was $10.74.  
This often happens because the hearing focus is upon the last thing heard, not the first.  The customer gave him a $10.00 bill and wondered why the clerk wanted more money.  The clerk said that the customer was WRONG.  The clerk said, "I said, $10.74 and you didn't hear me."  The customer asked for the details and the clerk read the bill to him, but didn't give him the receipt.  The customer said, "OK" and paid, but he had a quizzical look on his face.
  I think I would have handled this matter differently.  Wouldn't it have been better to say, "I miss-spoke, I meant $10.74."  In addition, I would have given him the receipt and carefully explained the total to the customer's satisfaction.  Then, I would have asked, "Did I get something wrong?  If so, I would like to make it right."
In this case, the customer was wrong, but one could have made the patron happier w/the result.  He would have  felt that he was treated politely and with sincere concern to make the sale to the customer's satisfaction.  How one says something is important in customer service, not just the process being technically correct.  One is right when the customer is right.

I recently visited Michigan State Univ. for NOAC w/7,400 others.  The OA Shows were great!  I was looking after some handicapped persons.  One of the them was a mother, about 40, who had a stroke.