Doctor Strange's lack of basic knowledge in his field
While performing a surgery, the surgeon needs to keep his hand clean, in order to ensure the safety of the patient. To do so, the surgeons wash their hands after getting fully ready. So, they put on their gear, including the mask, and then wash their hands. But Strange does the opposite.
At the very beginning of the movie, Strange has to perform a brain surgery. But, instead of putting on the mask and then washing the hands, he first washes the hands and then wears the mask. This leaves his hands open to contamination.
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