Ellen's hairstyle changes quite dramatically towards the end of the film (at around the time that Aunt Edna is dropped off at Flagstaff), from wavy to a poodle perm. Result of last minute reshoots.
When Clark turns in his old car at the dealer, his old car going into the cruncher is different than the one deposited back on the lot. Going in, it has stock hubcaps and black tires, and the smashed one has mag rim and a white-wall tire in front.
As Clark and Ellen enter the office, Clark opens the door for Ellen using his right arm. As the view switches to an interior shot of them coming in, Clark is holding the door open with his left hand, with his left arm over Ellen's head.
Through the course of the film, Rusty is about the same height as Ellen. But during the Walley World scenes, he is taller than her. This is because these scenes were shot after principal photography.
In the saloon in Dodge City as the Sheriff is walking up to the Griswolds inside the bar the waitress in the pink dress walks past the Sheriff from behind and stops at a table, and the girl in the dark red dress, coming down the stairs, walks past the sheriff in the other direction. In the close up shot of the Sheriff, both girls are walking into the shot from the same directions again.
On the Blu-ray Disc version during the "singing" sequence early in the film, the subtitles incorrectly refer to Clark as "Carl." This may be a deliberate reference to the goof of people getting his name wrong throughout the Vacation franchise, though.
When the Grizwolds are "lost" in St. Louis, the wide overhead shot clearly shows the Family Truckster exiting onto Interstate 70 West. This is the correct route for their trip, and they wouldn't have been lost if they took it.
When purchasing a car in Illinois, either your license plates are transferred from your old car, or you are issued a License Applied For sticker to be posted in the window of your car (at that time). The Griswold's old car is driven off and crushed without removing the plates, and there is no sticker on the new car. They would be stopped and ticketed by the first cop that spotted them.
When Clark and Ellen are doing the dishes, you can see Ellen just brush the leftover food into the sink and then hand the plate to Clark. She does not wash the dish. Clark is in front of an open dishwasher but he just wipes the dirty dishes with a towel and puts them in the cabinet. This type of subtle joke is a hallmark of Chevy Chase, and was done intentionally as a joke.
In Kansas when Rusty is asking his cousin Dale whether he had "Pac Man", Dale says "no", but in the very next scene in Dale's room, a portable Pac Man game can be clearly seen on the bed behind Rusty. However, this is Rusty's Pac Man game. He brought it along on the trip and it can be seen in the Truckster many times throughout the film. So Dale saying he does not have a Pac Man game is correct.
Clark comments on "real tomato ketchup," not "homemade," so seeing a bottle of commercial ketchup in Eddie's kitchen is OK.
Towards the end of the film, when the Griswold's reach Walley World, Clark tells Laskey that they have an appointment. When the shot switches to Laskey it's clear that he's "reading" off a blank legal note pad.
In the open matte version of the film, when Ellen is in the shower, she can be seen wearing slightly pulled down underwear or a bathing suit bottom.
When Clark falls onto the skeleton when wandering in the desert, a jaw spring can be seen in the back of the skull. This is used in the lab to keep the jaw shut, but obviously wouldn't be on a skull in the middle of nowhere.
In several frontal shots of the Griswolds driving along, there is no windshield on their car.
When the Griswolds fall asleep in the car, Clark awakes to see the predicament they're in. When his car is apparently in DRIVE, the car goes in REVERSE without stopping and/or changing gears.
It makes no sense that Ellen would put her credit cards in a travel bag rather than her purse.
Clark should have heard the howl's of Edna's dog while he was driving after leaving the campground.
When Clark and Ellen go skinny-dipping in the motel swimming pool, we hear her laughing and shouting when her head is underwater.
Right after Eddie tells Clark that Daisy Mabel was born without a tongue, we can hear Clark say, "Oh," and then laugh. However, Clark does not appear to be laughing, or even smiling.
Camera and operator visible in one of the rides at Walley World.
After Clark opens up the door just after the airbag deploys, you can see filming equipment in the reflection of the station wagon's metal side mirror.
Clark and the Motorcycle Cop are talking in close-up at the rear of the vehicle. A movie light is clearly visible reflecting off the cop's helmet as the sun is shining in a different place on the helmet.
On the pirate ship/swing, crewman with white shoes and jeans visible on the right side on the ground, maybe ride operator.
When Ellen sniffs the sandwiches and realizes the dog had peed on them, the lights used for the scene are reflected in her sunglasses.
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.
Tall mountains can be barely seen in the distance when the Griswalds arrive at and drive away from Cousin Eddie and Catherine's house & farm. There are no tall mountains in Kansas (the highest elevation is only over 4,000 feet above sea level in Wallace County, two counties to the North of Hamilton County, where the town of Coolidge is). The mountains visible in the background are in fact the Wet Mountains near Pueblo, Colorado, where the farm used to film Eddie's house was located.
Mountains and palm trees near Chicago, in the opening scene when they are pulling into the car lot.
As the Griswolds are driving through Chicago, operating oil derricks can be seen in the background outside the vehicle's passenger windows.
During the opening credits postcard montage, one of the cards shows a billboard advertising "Gatorland Zoo, 6 miles away", and the post card heading mentions the billboard was photographed at St. Augustine, Florida. Gatorland Zoo is located in Kissimmee, Florida, which is over 120 miles away from St. Augustine. A few postcards later, another card appears from Gatorland Zoo, depicting the "alligator mouth" entrance, and that card is correctly headlined as Kissimmee, Florida.
After Clark Griswold trades in his family's old station wagon, it is immediately taken to a junkyard and being crushed in the process and then later brought back to the dealership to surprise Clark when he insists he have the old car back while declining to buy the Wagon Queen Family Truckster they bribe him to buy anyways. This would take time to do as a junkyard is often in rural areas and at least a 10 - 15-minute drive from a dealership in urban places like Chicago, yet the scene taking place at the dealership is shown for 3-4 minutes screen time.
If Walley World was closed for repairs, then the entrance to the parking lot should have been locked, yet Clark drove through with no trouble.
Immediately before the Family Truckster crashes through the "road closed" in the desert, Edna can be seen gasping before Clark and Rusty, possibly a cue mistake. Following which, Ellen clearly can be seen laughing as she leans over for cover.
When the Griswold's arrive at Cousin Eddie and Catherine's, the Letter on their screen door has an "H". Cousin Eddie's last name is Johnson.
After Ellen announces that Aunt Edna is dead, Clark pulls into a ditch and Audrey jumps out of the car yelling "a dead person breathed on me!" Dead people do not breathe.