Alert the right person right away
When negative feedback hits the fan, the right person in the organisation needs to be alerted. That doesn't mean that just one person or a team of 'fire fighters' should be appointed.
Allow the problem or issue to be solved exactly where it occurred, be it in your point of sales, on the phone, online. Not only is it better for customer engagement, it is also beneficial for the employee. Let people learn from their own mistakes to turn something negative into something positive. The outcome will be marvellous: a happy customer and a proud employee.