Dawson Casting: 23-year-old Holliday Grainger plays Lucrezia, who's 14 in the first episode. 18-year-old Cesare is played by 25-year-old Francois Arnaud, 33-year-old Emmanuelle Chriqui plays a teenage Sancia, and 18-year-old Giulia Farnese is played by 29-year-old Lotte Verbeek (though this last may count as Age Lift; Giulia's age is glossed over to the point where the only thing made clear is that she is muchyounger than Rodrigo).
Giulia's almost definitely an Age Lift. It's made pretty clear that she's been married to her husband for a good long time—long enough to conceive a child and learn the ways of court and men, as well as politics, on some level. She's definitely not eighteen, and probably isn't supposed to be a teenager, either. It's likely that the characterization is a combo of the real Giulia and Adriana da Mila, Lucrezia's real-life governess/mentor-type figure and a cousin to the Borgia family.
Fake Nationality: Hoo boy. We have Brits playing Italians (Derek Jacobi, Sean Harris, Julian Bleach), Brits playing Spaniards (Jeremy Irons, Holliday Grainger, David Oakes, and Joanne Whalley), a Brit playing a German (Simon McBurney), a Dutch woman playing an Italian (Lotte Verbeek), a Canadian-American playing an Italian (Colm Feore), a Lithuanian playing an Italian (Ruta Gedmintas), a French-Canadian playing an Italian (Emmanuelle Chriqui), and a French-Canadian playing a Spaniard (Francois Arnaud).
Lying Creator: If you look at the record, the show runners were this in spades. In two of the more notable examples, they said that they wouldn't go into the Banquet of the Chesnuts because it would be too much "even for Showtime." They did. Then they said they wouldn't go into the Cesare and Lucretzia incest before doing it as well.
Screwed by the Network: Showrunner Neil Jordan had originally planned for the show to last for four seasons like The Tudors. So when Showtime announced that season 3 would be the last, there was an outcry from fans. Jordan later revealed that he had proposed to make a two hour film to wrap up the storyline and give the show a Grand Finale, but Showtime declined, citing high production costs.
Both Jeremy Irons (Rodrigo Borgia) and Colm Feore (Cardinal della Rovere) have played Tom Hiddleston's father at one point or another- Irons as Henry IV to Hiddleston's Prince Hal in the BBC's 2012 adaptation of William Shakespeare's Henriad, and Feore as Laufey to Hiddleston's Loki in 2011's Thor.