When Melvin manages to get his car out of the mud, in the next scene is not that dirty at all, even though he was stuck in the mud and the car should be dirty.
When Melvin drives his car through water, it obviously splashes and shakes the camera.
When Melvin drives his car around Louisiana the radio news broadcasts he hears are from Toronto, Canada, referring specifically, for example, to "The Hospital for Sick Children" located there.