A complaint is a customer saying something went wrong. How it is handled determines whether they stay, and whether it becomes public.
Complaints are information
Most dissatisfied customers say nothing and simply do not return. The one who complains has given the business the chance to fix it and the information to prevent the next one.
Patterns in complaints identify where the process is failing, which is otherwise invisible — see quality control.
The sequence
Acknowledge quickly. Delay converts a complaint into a review, because the customer escalates to somewhere they will be heard.
Establish what happened, from the customer and from the records. Photographs at completion frequently settle it immediately.
Say what will be done, and by when. Vagueness is read as evasion.
Do it, and confirm.
Where it is the business's error
Fixing it without argument is almost always cheaper than the alternative. The cost of the return visit is known; the cost of a public review is not, and it persists.
Where it is not
The customer still needs an explanation they can accept. Where the work was outside scope or the fault was not the one reported, that is a conversation supported by what was documented at the time.
This is the practical reason scope and exclusions are written down.
Public responses
A reply to a negative review is read by prospective customers rather than by the complainant. Measured, factual, and offering to resolve it offline reads well. Arguing does not, regardless of who is right.
The recovery effect
A problem resolved well produces a more loyal customer than a job with no problem at all, because the customer has now seen what happens when something goes wrong.