In the theatre alone there are at least half a dozen people. At any stage an error could affect the quality of the care John receives. Because of time pressure, the admitting doctor may fail to record his allergy to penicillin. The design of the medical record itself does not allow this omission to be picked up. In the theatre there may be conflict between any of the participants with the potential to distract attention from the patient.
Individuals have their own ritualistic tasks pre-determined by their profession.
The skill of the doctor doing the hip replacement is only one component. This is not uncommon. The doctor who gave the injection is blamed. There are instances where an individual's competence or conduct is the main cause of a patient's suffering, and they should be held professionally accountable, but in most cases the cause is a combination of factors, not just the actions of one person. Tribalism also determines how the system responds to an error.