Revealing mistake: At the gas station, while Clark watches the mother and son refuelling the car beside him, she points at her son to remove the gas cap. If you watch closely, he looks like he unscrews it but he has nothing in his hand, and when he "places" it on the rear bumper, in the next shot there is nothing sitting on the bumper.
Continuity mistake: When Christie Brinkley first drives alongside the Griswold car, the speedometer on their car is shown hitting 75. But when we see Christie's point of view of the car, it's clear from the trees in the background that the car is going about 35 mph. (00:26:10)
Factual error: The whole movie he drives without any valid license plate. Just a crooked colored plate from the auto store. He would have been pulled over for that immediately, and the car probably would be impounded. Then when he does get pulled over for the leash, the cop says nothing about invalid plates. But he asks for registration proof and Clark shows it, even though he has no legal plates. Plus his meaningless plate is crooked, which is also illegal.
Continuity mistake: When they are on the looping roller coaster, they come to a point where it is a long drop in to the loop. However, in the next shots of Clark and the security guard, the coaster is going round a curve, and in the next shot it is back on the straight drop.
Continuity mistake: When the family finally makes it to Wally World, they park at the back of the lot with the car facing the park. As they pull up the shadow of the car is on the passenger side (right side), however only moments later as the family is running toward the entrance their shadows are on their left. If you look closely you can see that they are running from the front of the car, and the shadows should still be to the right.