Já agora aí vai um dos paradoxos de um Regresso ao Futuro

In 1966 Jennifer, a girl of 16, meets a vagabond, Roger, along the road. They start talking, and after a while Jennifer seems to be pregnant. Roger disappears without her knowing his name. Nine months later, due to complications with the birth, Jennifer needs to change sex. Her child is also robbed from the hospital. Twenty years later Jennifer, now known as Roger, is poor and survives as vagabond. In 1986 in a bar, after a couple of drinks, Roger does his story to the bartender. This one has got an interesting proposition: he gives him the opportunity to travel back in time and take revenge on the vagabond that made him/her pregnant. Therefore he has to join the secret organization of time travellers. Roger accepts, but when he arrives in 1966, he meets a girl, Jennifer. He makes her pregnant, doesn't find the vagabond and begins a bar. He joins the secret organization of time travellers and talks to the vagabond in 1986. The bartender disappears and travels to 9 months after 1966 to steal Jennifer's child, a girl, out of the hospital so Jennifer doesn't have to raise it on her own. He brings her 16 years back in time, to 1950, where he leaves her in an orphanage.
A lovely paradox, because Jennifer is her own mother, father, grandma, grandpa, son, daughter, grandson and granddaughter. If we ask: Where does Jennifer come from, we see that this paradox is an excellent example of how time traveling doesn't change the past, but completes it. It's interesting to notice that even time traveling can't prevent you from dying and getting born. Jennifer gets born, and she dies as Roger, but she's captured in a vicious circle.

in http://users.pandora.be/vannoppen/science5.htm