In the scene where Ferris tries to rewind the car's odometer, he experiences no success in getting it to go backwards by sticking the car in reverse. The writers are dead correct: in a car with an electrical odometer, you can't rewind it like that (or at all, it's one of their best features!).

The problem occurs because the car they use in the movie is fitted with a standard MECHANICAL odometer - which, as our good friends at howstuffworks.com can correctly tell you, are quite susceptible to this kind of misuse. The odometer SHOULD have gone backwards, just as predicted - and the fact that they bothered to rig this up and then say it wouldn't work ranks this as a particularly big slip-up.