Ok, it was a genuine mistake, and unfortunately the customer was unhappy enough with the situation to have dicontinued with the service. It doesn't make them a bad customer, just a dissatisfied one.
How many of us on here dump a customer with whom we are dissatisfied? It may come as a surprise to some, but we can't reasonably expect all customers to dance to our tune, then bleat about it when the tables are turned.
Just don't take it personally. We win some, we lose some.
John