Revealing mistake: In the film's first scene, as Clark and Russ Griswold make a left hand turn to pull into the used car lot in Chicago, the upper quarter of the screen shows mountains and palm trees in the background, which is an impossibility for Chicago. I believe the nearest mountain range to Chicago is in Arkansas and the nearest palm trees reside in Louisiana. (This scene was filmed in California, which explains the mountains and palm trees, BUT nonetheless a mistake.)
Revealing mistake: In the scene where Chevy Chase is walking through the desert searching for a gas station he falls down and ends up staring at a skull next to a small fuel tank. If you look closely, you can see two springs on both sides of the inside jaw. This is obviously a skull model and the springs are there to keep the lower jaw shut tight against the upper jaw.
Factual error: After they leave Dodge City, Ellen says they should stay on US-50. US-50 in Kansas is not a 4 lane, divided highway. In the same scene, as Christie Brinkley approaches in her Ferrari, there are mountains visible in the background. There would be no mountains visible anywhere in Kansas.
Revealing mistake: In the scene where Clark ties the dog to the bumper of the family truckster, Rusty hands the leash to Clark while the dog is chewing on his leg. The camera changes to show Clark's face close up, and when he pulls the leash to free his leg from the dog, you can see the end of the leash fly behind him (with no dog attached.)
Revealing mistake: In the scene where the Griswolds tie Aunt Edna to the roof of the car in a sitting position and drive her body in the pouring rain, pay close attention to the back of her head (which is not covered like the rest of her body). It's easy to see that it's a mannequin, with the pure-white plastic skin. Also, the mannequin's hair is very short in the back, whereas Aunt Edna's is longer.
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.
Other mistake: During the scene when Clark is driving his family through the "rough part" of town in St. Louis, Clark asks his children if they are noticing all of the plight. Clark then makes the comment, "This will just make us appreciate what we have." A gunshot and a scream is heard, and Clark exclaims, "Roll 'em up!" A window is visible on Ellen's side of the car as she is rolling her window, up . But, there is no window on Clark's side of the car. Clark goes through the motions of rolling up his window, but . In actuality, he is rolling up nothing. Chevy Chase mentioned this during the cast commentary for the film.
Factual error: During the St. Louis part, they show the Griswolds crossing the bridge into Missouri and *then* getting lost in what they title "East St. Louis." East St. Louis is actually in Illinois, so they would have encountered it before crossing the bridge. This has apparently been corrected in the video version.
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: At the beginning of the movie, when Clark says he's not falling for the salesman tricks, Clark and Rusty start to walk back to their old car. Before the next shot, you can see Rusty walk past the back of the car and almost make the turn to the passenger side of the car. In the next shot, you see Clark trying to open the door while Rusty just starts walking up to the driver side of the car.
Plot hole: When the Griswolds are at Wally World, supposedly the only people there are the guards. Who operates the rides for them?