Irate or irrational customers will often judge your level of service based on how you respond to a mistake. Do it well and they will probably forgive you and possibly even say positive things about your business to other people. That is a very good thing and what you want customers to say about you.
How To Handle Difficult Customers
You always want to provide exceptional service to all your customers. However; in the real world, things go wrong and mistakes are made. People come in to your store in bad moods and react not in the moment but from something else they are carrying with them.