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 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: 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.
Plot hole: When the Griswolds are at Wally World, supposedly the only people there are the guards. Who operates the rides for them?
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.
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. (00:13:50)
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: 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.
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.
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: 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.
Revealing mistake: There is a spring in the skull in the scene where Clark is roaming through the desert and falls down by the skeleton with the gas can.
Visible crew/equipment: On the second roller coaster in Wally World, as they begin leaving the station and going up the lift, crew can be seen crouching in the background trying to hide.