Haven't you watched "House"?
....your point is taken, but I think there are more checks and balances in the system than you realise, particularly as the complexity/severity of the condition increases. For a simple problem, there may be one medical opinion (eg a primary care physician) which is unverified and treatment advice goes unchallenged. Prescription errors will be counterchecked by pharmacy, but they cannot question the original diagnosis.
In a hospital, patients (by definition iller and potentially more at risk of serious disease and harm) are seen and reviewed by a succession of ever-more experienced doctors. There are ward pharmacists who check prescriptions and may even have prescribing rights of their own. There are usually multidisciplinary team (MDT) meetings about complex cases, specialist teams look at individual patient aspects of care (eg the microbiologists or diabetic specialist team) and at least once a week patients are discussed at MDTs (eg radiology/oncology/chest medicine meetings) usually with a clear consensus view arriving from the input of several experienced clinicians. A patient in ITU may get a "second opinion" every 12 hours, as the consultant responsibility is handed over each shift.
That's not to say things work well all the time and that diagnosis is foolproof - it's not. (and then there is always the risk that the radiologist has opted out of doing anything that day because he felt tired and didn't want to risk making an error and being accused of misconduct...

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