There are times when Murphy's law does it's thing and there is not much you can do about it. But could this be an oppurtunity to show how much your business really cares/values them as a customer. Yes!

Put yourself in their shoes and really think about what it would take to make it right it this mistake happen to you. Now, whatever you come up with, double it and do that for them.

Genuinely exceeding their expectations is the key.