Revealing mistake: In the early part of the film, Beverly D'Angelo and Chevy Chase check into the hotel/motel. She goes in to take a shower but he barges in. If you look real closely, she is wearing underpants and is not nude although she is taking a shower.
Revealing mistake: In the scene where Christie Brinkley takes off her clothes and invites Chevy into the pool, he takes off his pants and we are supposed to assume he's naked. When he dives in you can see he's wearing flesh coloured underwear.
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.
Continuity mistake: After the car is fixed in the desert, the tires all wobble. But when the are leaving the Grand Canyon after Clark steals the money, the tires ride straight again.
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.
Continuity mistake: When the officer stops Clark about the leash, Clark gets out of the car and is talking to the officer. If you look into the car when they show Clark nobody is in the car, but Aunt Edna, Ellen, Audrey, and Rusty are all on the vacation.
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: 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.
Revealing mistake: Aunt Edna is supposed to be dead. In the scene where the kids keep pushing her back and forth before they know she's dead, look closely at her eyes. You can see them flickering rapidly while her head is against Rusty's shoulder.
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?
Revealing mistake: John Candy reviews sheets of paper on his clip board, twice, to see if Clark Griswold's name is there. The clipboard holds only blank legal paper.
Continuity mistake: When Christy Brinkley appears in the Ferrari and Beverly D'Angelo is asleep in the car, Beverly's head is facing towards Chevy Chase. When they go to a shot of Chevy, Beverly's head is facing the other way. This continues back and forth throughout the entire scene, although she moves some.
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.
Other mistake: When Clark falls asleep at the wheel, right after the guy with the grocery bags moves out of the way, if you look closely in the driver's seat, you can see a man with his hands firmly on the wheel, fully awake. Right before this happens, the focus is on Clark sleeping.
Continuity mistake: During the campsite scene, the kids take the dog up to see the pool, the scene after has Clarke and Ellen finding their cabins. In the background, you can see the pool with the kids standing there, but the dog is gone. You can see there is no dog when they start to walk down the stairs.