Everyone can have a problem or a business difficulty arise. It happens, but after it happens what can make or break your reputation is how you resolve a problem. Consider problems a learning experience.
When a problem arises, for you as the business owner your best tactic is to listen actively to a complaint and then to find out what it is that the customer feels is fair to resolve the issue. Sometimes to offer a refund is the best approach and sometimes it is best to redo whatever was an issue. Either way a prompt and honest response is needed. There is nothing worse than a customer who has had an issue and then waited and waited for a phone call. Quick assistance can put out a fire rapidly.
Having a signed letter of agreement before you start work is also extremely helpful as this prevents situations where a customer states you said you would do this and you did not. If you spell out what you do in the agreement and both parties sign the agreement the additional request is handled as a change order with an additional charge or a comped charge if you are so inclined.
If you have made a mistake in handling a customer complaint, sometimes there is no way to go back and fix the problem once it has happened, but you can make sure to learn from it. Several times in my career I have said to myself “well, that didn’t go so well. Note to self, don’t do that again!”. Use these situations as opportunities to learn from a mistake and move forward building new customer focused strategies for the future.