Revealing mistake: When Clayface disguised as Joker is holding Talia hostage in the movie theater, they are both standing on stage in front of the screen with a movie playing. However, they are not casting shadows on the screen. Also, no shadow appears on the screen when Clayface takes on his true form.
More mistakes in Batman: Arkham City
Batman: It was all a lie. There's nothing wrong with you.
Joker: Nice of you to say. But you of all people should know there's plenty wrong with me.
Trivia: When doing the side-mission "Enigma Conundrum", after going to where the first Riddler room is located, look up and you will see a sign for a place called "The Black Canary." The Black Canary is another DC Superhero and member of The Justice League.
Answer: Listen to the fourth Riddler interview tape. He did a psychological profile of Batman, then picked among candidates as to who could be Batman. He's also smart enough to realize only a rich man could afford all of Batman's gear and vehicles.
Rob245