The studio planed for this to be a large franchise like The Hunger Games (2012) or _Twilight_, but due to poor box office, critical reception and fans of the books being disappointed, the idea was scrapped and it was decided for the story to be rebooted for a television series.
In the books, The Silent Brothers are described as having a much more gruesome look (eyes removed, mouth sewn shut and scars on each cheek), therefore they talk through minds. Although in the film you sense they are not talking like normal persons, it makes no reference to the fact they do not speak aloud as mouths are sewn shut.
The Mortal Instruments: City of Bones (2013) is supposed to take place in 2007, which was the date of the first book's publication. In the movie, Clary has an iPhone 4, but the iPhone 4 wasn't released until 2010. The characters should have had first generation iPhones, as no other iPhone models had been released at the time.
Warner Bros. originally had the rights to The Mortal Instruments. Kulzer speculates that they were looking for the next Harry Potter, but had trouble with the female lead character and eventually let the property go. Kulzer grabbed the rights and took the movie to Sony.
In the movie, Clary notices that Simon was bitten by vampires and he starts showing signs of turning (he doesn't need glasses). However, in the book, he swallows vampire blood while being a rat and, later in the series, he is bitten by the entire clan and is then turned.
In the book, Clary plans to go to pandemonium (the night club) with Simon and they get in because they act like they should be there. The demon boy who is killed, is lured into a closet by Isabelle and killed out of the way, but Clary walks in and sees what they're doing. In the film, the demon is killed in the middle of the club because no one can see them. This is where Clary finds out a lot of information about demons, given to her by Jace although Alec objects.