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I am using House MD as an example, but this is not specific to that show.
There are scenes where patients get injected something, or they have blood taken, or they get a small cut when operated. Another example might be when they intubate a patient.
I don't know if any of this might be potentially harmful, but I think the cost of actually doing it might be lower than faking it with CGI.
Note that, as I'm saying in the title, I'm talking about harmless scenes, not scenes impossible to film live, like surgery.