How rare are error coins in US Mint Proof sets? I received a set for Christmas and 1 coin appears flawed.?

The Kennedy 50 cent piece has a bright, shiny Propecia buy dot in the matte finish area of the neck on the profile side, pretty much in the dead center. I would say its 1-2 millimeters in diameter. I compared it to other proof sets and they do not have the spot.

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