Revealing mistake: In the car chase scene where the cop is after Roland, you can see that there is no key in the ignition of the cop's car.
Revealing mistake: After the girl is shot through the window and Tyler is beating on the sniper, he hits him with the gun. You can see the gun bend and shake, showing that it's rubber or another soft material. Even better when viewed in slow motion, it's very easy to see how much it bends. (00:42:05)