Deliberate mistake: When Marge goes offroading to get home there is a grassy field which leads right up into the Simpson driveway. However, in all other episodes there is no grassy field and instead there is a house in the spot where the field would be.
Other mistake: When Marge takes Homer back inside, his car is seen blocking the traffic. But at least one side of the traffic is driving on the wrong side of the road. The drivers behind Homer's car are on the right side of the road. And the drivers behind Homer's car from the opposite direction are driving on the left side but facing the drivers behind Homer's car on the other side. Either way, one half of the traffic are driving on the wrong side of the road. (00:02:35)