well the chemical in monobenzone isnt hydroquinone that you and I can get at the drugstore. Its another chemical (or a "cousin") of hydroquinone called monobenzyl ether hydroquinone. If you use mono at high strengths for a long period of time you could get chemically induce vitiligo (white spots)
Other then that if you are using ANY lightening cream and you dont apply it evenly then you can get lighter spots, or just uneven lightening.