You mean the card that would have been the BB's first downcard? This isn't specifically called out in RRoP, but if I were dealer I'd confidently declare "misdeal", pick up sb's card and shuffle -- the same as if I'd flipped it up accidentally.
By the letter of RRoP, treat it as a scrap of paper. Note that if a 2nd boxed card is found before action starts, it will be a misdeal.
I'm quite a fan of the "worthless piece of paper" rule, because it includes text that says things like "worthless piece of paper" or "worthless scrap of paper."
I'm quite a fan of the "worthless piece of paper" rule, because it includes text that says things like "worthless piece of paper" or "worthless scrap of paper."