Other mistake: In the scene where they surprise Big Momma for her birthday, Nolan takes the gun from Malcolm (he says give it to me, you ain't supposed to have that, etc. then says I'm gonna go secure the perimeter when he gets the gun) . They show the gun and their hands up close. It is loaded, the safety is off, it is cocked and Malcolm has his finger on the trigger. Nolan is pressing Malcolm's fingers. If he had really been fighting for the gun the way it seems, the gun would have gone off.
Other mistake: Near the end where the real Big Momma knocks over the 2 fighting guys and falls back on top of the security guards, and she's being held down, you can see what appears to be part of a fat suit coming from underneath her clothing. (01:26:50)