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 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: 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.
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.
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.
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.
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.
Revealing mistake: When they are riding the rides at the end, someone has to be starting and stopping them.
Revealing mistake: Aunt Edna is supposed to be dead, but look closely as she's sitting in the pool chair, and you can see her upper body move slightly.
Revealing mistake: Christie Brinkley supposedly jumps in the pool naked but she's wearing a flesh colored top that can be seen in several shots.
Suggested correction: No they show he is never naked, absurd post.