You don't have to tip for bad service.
Roy has had a very bad experience at a restaurant because the server is careless as well as rude.
Roy wants to express his discontent about the service.
Roy complains about the service to the manager and doesn't leave any tip.
Roy mentioned his dish made prepared incorrectly but leaves a small tip anyway.
The workers are sent a message that the service was exceptionally poor.
The server does not realize or care about the extent of the service failure.