During take-off, the PAN AM plane shown is a cargo plane, with no windows. In the next scene, Clark sits in a window seat.
The first time the Griswalds drive their yellow Austin Maxi, Clark drives on the wrong side of the road and hits another car. The yellow Maxi's left front fender is badly damaged. In subsequent shots, the damage is much less severe.
Clark gets into the passenger seat, thinking it's the driver's seat. When he and Ellen change places, she leaves the door open. The door is closed when Clark reaches that side of the car.
When the Griswalds are stuck on the traffic island near Parliament and Big Ben, the long shot shows a Chrysler 180 hitting their Austin Maxi. In a close-up the other car is a Morris Ital.
When Clark mixes up the English hotel rooms, the door on the left leads to the lobby; the one on the right leads to the strange woman. After he meets the strange woman, he runs out and meets Ellen in the hallway, but he emerges from the lobby door, not the strange woman's door.
Rusty suggests the family go to the top of the Eiffel Tower. When the Griswalds are at the tower, the down arrow in the background means they are clearly on either the second or first level. The elevator is the only way to the top, and the observation deck is not that big.
When the family is at the museum in Paris, the soundtrack plays a Belgian song---Ça Plane Pour Moi---by Plastic Bertrand, the one-hit wonder from Belgium. This is the equivalent of France making a movie about a vacation in USA and playing the Canadian anthem as background music.
Eric Idle's character is shown collapsing in the fountain. It's not made very clear that this is because his leg plaster has dissolved.
Clark calls the street in Germany "Dipplestrabe" with a B sound. In reality, the German character which looks something like a B, represents a sound "ss". This is almost certainly deliberate, to show that Clark is an ignoramus.
During Ellen's dream on the plane of the Queen's reception the guests are greeting the Queen, Princess Diana, and Prince Charles and then turning away. You always face the Queen and never turn your back to Royalty in Great Britain. This is probably deliberate to show how uncultured the Griswalds are.
In the Paris hotel room, Clark flops onto the bed and falls off. He quickly gets up and sits on the side when Rusty appears to talk about acclimating to France. Suddenly out of nowhere, Clark produces a set of berets from behind him for the family to wear.
During the chase scene, when Clark jumps into the car from the bicycle, the bicycle remains straight, as if it were attached to the car.
The Vacation series is about a family named Griswold. In this movie, whenever the name appears in print, it is spelled Griswald.
When Chevy Chase accidentally hits Eric Idle's leg on the revolving door, Idle screams in pain well before the door hits his leg.
The chase scene uses obvious stunt doubles for Clark chasing after the bad guys on the bicycle and in the car, and the bad guys in their car.
Visitors to Stonehenge are not allowed to get near the stones and can only see them from a distance, so Clark could not have caused the destruction that he did in reality.
In the beginning, the plastic container of lighter fluid makes a metallic sound when Clark and Ellen squeeze it.
The Griswalds stay in a very tiny hotel room. When the camera pans up towards the ceiling, the set structure holding up the wall and ceiling are visible.
When Audrey calls Jack from the London hotel, she dials 7 numbers before Jack answers. Calling the US from the UK requires 13 numbers (001 + Area Code + 7-digit number).
When the plane passes the Statue of Liberty, the passengers look out the window on the left, but the view shown is on the right side.
When the Griswalds abandon their car in Germany, they also abandoned their luggage. Yet when they are on the train, they have changed clothes.
When the thief takes the Griswalds' picture in front of the fountain in Paris, Ellen holds up the Paris brochure. The flag on the brochure is green, white, and orange, the colors of the Irish flag.
The "Pig in a Poke" host asks the Griswalds about the leader of the 1804 American Expedition, leading them with "What Lieutenant lead that expedition?" Ellen Griswald inadvertently answers by saying Clark's name. The actual answer is "Lewis," who was a Captain. William Clark was also a Captain, but Meriwether Lewis was the leader, and Clark was a follower. Therefore, the Griswalds would've gotten the answer wrong, lost the game, and the movie would've ended right there.
The "Pig In a Poke" announcer says they won a "supercharged Honda 750 motorcycle." The motorcycle shown is a Honda Rebel 250.
While reading the Pig in a Poke itinerary on the flight to London, Clark is holding it upside down.
When the Griswalds are getting their picture taken in front of the fountain, as Clark is showing the thief how to use the camera, the thief's shirt has bunched up at the bottom. You can see Clark straighten his shirt as he moves around to the front of him.