Continuity mistake: There is a scene where Arnie and Danny DeVito are in the washroom of a nightclub. In one shot there is a beer bottle on top of a hand dryer, and in the next shot it's on the other hand dryer.
Other mistake: When Devito sends Arnold into the house to get his stuff and go with him to New Mexico and Houston, Devito quickly jumps into the car to drive off. (You can see on the long street behind him that there are no cars coming). Yet, not one second has gone by and you are looking at him from the front of the car and Linda and her sister pull up behind him blocking him in the driveway. They turned in from the left which was the same stretch of street you were looking at with no cars on it just one moment earlier.
Continuity mistake: In the scene where Danny and Arnold are eating the "nuked" food. You can see the amount of milk Arnold has in his glass changes. He actually has more at the end of the scene.
Audio problem: In the bar scene where the twins are dancing with the girls, keep an eye on the band in the back. The singer and the guitarist are not always singing and playing in time with the music. Most visible at the end of the scene, the music has the guitar playing a long riff and no vocals, yet the guitarist is barely moving, and the singer is singing with emotion.
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: 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.
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.
Visible crew/equipment: When Arnold throws one of the Clane brothers into the elevator, you can clearly see the cable that pulls him backwards.
Factual error: When Arnie and Devito are stealing the car and Devito sets off the car alarm, Arnold picks up the car from the back end to stop it. If you watch the wheels you will notice that they don't shake or move at all when Arnold sets the car back down on the ground.