Casey argues with Lucy that she is 9 years old, when Lucy says that she is 8. Although the movie is trying to show how the parents neglect to remember her birthday, given that her date of birth is December 11, 1975, the Casey character would still be 8 at the time the movie was released.
Although Lucy and Albert meet in early 1973, Albert says that Casey was born the following winter during December 11, 1975. The following winter would have started on c. December 20, 1973.
When Lucy and Albert pull over on the side of the road after shortly arriving in Illinois, the car is on a slight left to right incline. When they are filmed in close-up inside the car, they are shown still in a horizontal position.
When Albert, Lucy and Casey are filmed through the front windshield of Albert's Cabriolet, the rear-view mirror is missing. The rear-view mirror can be seen in all subsequent shots in that sequence up until they park at the restaurant, Albert actually looking at Lucy seated in the back a few times in the mirror.