Other mistake: During the scene when Clark is driving his family through the "rough part" of town in St. Louis, Clark asks his children if they are noticing all of the plight. Clark then makes the comment, "This will just make us appreciate what we have." A gunshot and a scream is heard, and Clark exclaims, "Roll 'em up!" A window is visible on Ellen's side of the car as she is rolling her window, up . But, there is no window on Clark's side of the car. Clark goes through the motions of rolling up his window, but . In actuality, he is rolling up nothing. Chevy Chase mentioned this during the cast commentary for the film. (00:13:50)
Other mistake: When Clark falls asleep at the wheel, right after the guy with the grocery bags moves out of the way, if you look closely in the driver's seat, you can see a man with his hands firmly on the wheel, fully awake. Right before this happens, the focus is on Clark sleeping.
Other mistake: In the first gas station scene, Ellen directs Clark to the gas tank opening which Clark has thus far been unable to locate. Despite being a brand-new car, the inside of the gas tank lid is completely rusted.