Since the deck is considered to be random in terms of both content and order the error can be completely unwound. There is no reason why either player should be advantaged or disadvantaged by an accidentally revealed prize card during set up. This would not be the case if the deck was considered to be random but possessing a yet to be revealed order.
Along with the caution/warning, I'd return the revealed card to the deck, shuffle it, and then continue to place the prizes.