When making up his mind about a social system (laws, taboos, customs, whatever), Bob is likely to be biased by where he expects himself to be placed in this system. Of course, his daydreams are likely to turn into nightmares as he gets to know what it's really like on the other side and he already made a moral commitment to principles that are now to his disadvantage. He would have been wise to think in the same way as John Rawls' thought experiment about the "original position". But now it's too late for that. Bob has already painted himself into a corner. If another character actively manipulated him to get there, this can be called an Original Position Gambit.