Revealing mistake: The diary page you retrieve while Saori is unconscious with the Crow knight still in the stadium has just the same sentence repeated over and over.
Other mistake: In the Sanctuary invasion side story, Carlos warns that "The Sanctuary has been invaded. There peace of the world has been broken", instead of "the peace." There are other words blatantly wrong, such as, when talking to the priest "Let's see of your heart is sincere enough", instead of "if."