Other mistake: In the scene where Mary Lynn Rajskub is driving Harrison Ford around, she is told at the beginning of the scene to "pop the clutch", signifying that she is driving a manual transmission vehicle. However, during the drive, she never removes her hands from the steering wheel to shift gears.
Deliberate mistake: When Harrison Ford and Mary Lynn Rajskub are crossing the tracks, its Rajskub who's driving the car, not Ford. This is because the filmmakers originally had Rajskub driving the entire way. They later refilmed the drive with Ford driving, but kept the shot of them crossing the tracks the same. Ford and director Richard Loncraine confirm this on the special features of the DVD.