Continuity mistake: In the Italy scene after the visit to the Traveler's Cheque center, the Griswold's decide to go shopping with the $3000 they received. Rusty proclaims "Yeah Dad, we've been in these clothes a long time, I feel rank." Shortly after walking out of the store wearing their new clothes it changes to the scene right before they see Ellen on the billboard, and they are back in their old rank clothes.
Continuity mistake: When Clark first hits the bike rider with his car, he is nursing his left leg, but later, at the hotel in Rome, he has a cast on his right leg. (00:26:50)
Continuity mistake: When the Griswalds are leaving the hotel in London they get in the hire car. When Clarke realises that the steering wheel is on the other side he gets out. When Ellen gets out she leaves her door open but when Clarke goes to get in the door is closed. (00:24:20)
Continuity mistake: Near the end of the film as the Griswolds flight back to the U.S. approaches New York City, Clark points out that the Statue of Liberty is visible below. The Griswolds are all seated facing forward (in the direction of flight) on the left side of the aircraft, and are looking down on Lady Liberty through windows on the left side. When we see the subsequent shot of the statue passing by below, the point of view is from the windows on the right side of the aircraft. (01:30:05)
Continuity mistake: When Ellen is dancing for Clark, she slides down against the door. There is pink toilet paper by her feet which disappears.
Continuity mistake: This is the only movie in the Vacation series where the Griswold name is spelled "Griswald." This is shown on the beginning credits on Clark's passport as well as family name on Pig in Poke game show.
Answer: The answer could be any of those suggestions. It could also very well be a movie mistake. It's not uncommon for character names to be misspelled, either on some document or in the cast credits.
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