Revealing mistake: When Two Face and Riddler enter Wayne Manor, and knock Alfred down. Alfred, unconscious, pushes the dinner tray aside, so the thugs can carry him into the closet. (01:27:55)
Revealing mistake: When the Riddler projects the question mark above the bat signal, two beams can be seen producing the image, except they are slowly tracing the perimeter. If these beams are producing the image, it should only be visible where the beams are touching, and not everywhere else. In order to produce an image and not just two points, beams would need to travel extremely fast to trace the perimeter without leaving gaps. (much the same way images in a lasershow are produced). (01:33:10)
Revealing mistake: When Dr. Meridian first drops out of the tube, her skirt flies up. However, when she is falling in the shaft, her skirt is down and barely moving. (01:46:55)
Revealing mistake: When Batman is hanging from the chain that is connected to Two-Face's helicopter, you can see a dark area at the bottom of the screen. The dark area is a shadow, and you can tell it's green screen. And yes I know he's hanging off a helicopter.
Chosen answer: Batman talks like that because if he talks like Bruce Wayne (Val Kilmer's voice) his voice might be recognized, leading to Batman's identity being revealed.