Forgot the name of a movie/show

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There was a movie or a show where someone goes in to have a product tested on them to make money. One of the guys administering the test blows dust into the persons eyes and their vision disappears. The person then asks the two guys in white coats administering the tests "doctors, when will my vision come back". One of them replies "oh, we're not doctors" while laughing and says "this guy thinks we're doctors". or something along those lines. It was really funny I thought but I for the life of me can't remember what it's from. Can't even remember the actors in it. Totally blanking. Anyone have a clue what movie or show it's from?