Continuity mistake: Arnie & DeVito are walking through a crowd with their white coats hanging over their shoulders, black sunglasses and hawaiian shirts on (DeVito is teaching Arnie how to act semi-normal). If you look in the near background you can see a few extras that walk across right behind them about seven or eight times, they were obviously told to walk closely across behind the two stars and then turn around and walk closely across behind them again and again and again. You would think that a movie with such a huge budget would be able to hire a few more extras to do that job....
Continuity mistake: Arnie enters the parking lot driving on two wheels crying "Woooa." The close shots show him with one hand on the wheel and the other outside on the window; but on the wide shots, with the stunt, both hands are inside and his mouth closed but still the cry is heard. This mistake goes on every time angle changes.
Continuity mistake: At the end of the film, when they are at the fair with their own "Twins" Danny is holding his children and the one in his left arm is crying while the one in the right is not. When the snapshot is taken and the picture shows the still shot, the crying baby is now lethargic while the other is crying instead.