When the family of J Paul Getty II goes shopping in San Francisco in what is supposedly 1964 according to the captions leading up to the scene, there is a maroon first or second generation Mercury Cougar in the parking lot of the store. The earliest Mercury Cougars were released for model year 1967.
Also (this is not a goof), one wonders how a family that is supposedly tight financially could have an eight year old Chevrolet Wagon in such straight and clean condition with no faded paint, dents or missing parts visible.
When Gail is showing Fletcher Paul's room, a poster for the movie "Lo straniero senza nome" can be seen on a wall. It's the Italian title of "High Plains Drifter" by Clint Eastwood. The movie was released in August 1973, while Paul Getty was kidnapped in July 1973.
When Gail & Chase are on plane and about to land in London, the captain can be heard saying that the temperature is 60 degrees Celsius which would be 140 degrees Fahrenheit. That would be considerable hotter than the hottest day ever recorded in England which was 101.3 degrees in August, 2003.
In the early scene where the family has breakfast there is a glass bottle of juice visible, which change amount of juice back and forth between each different camera angle, less juice, full juice, then less again, then back to full etc.
The end of the film depicts J. Paul Getty dying at the same time Fletcher Chase and Gail Harris rescue J. Paul Getty III. In reality, the elder Getty died in 1976, three years after the kidnapping of his grandson.
The Carabinieri attacking the kidnapper's cove are seen handling a number of different firearms. One of the them is a Beretta AR-70/90 assault rifle. This is an assault rifle introduced in the 80s and used only by the Italian army proper, not the Carabinieri.
The villa model J.Paul Getty is showing Fletecher near the end of the movie is the Getty Villa in Malibu. This villa was not based on the Villa Adriana at Tivoli, that is shown being visited by J.Paul and his grandson at the beginning and mentioned again by J.Paul to Fletcher. It was in fact a reconstruction of the Villa of the Papyri of Herculaneum. This smaller mansion was destroyed in the same eruption that destroyed Pompeii in 79 CE, and was not a imperial residence, but probably the property of a well to-do Roman patrician.
11 minutes into the movie, in a scene that occurs during a period identified as 1964 and showing per-Christmas preparations, we see a snow storm in San Francisco, CA. The only snow in San Francisco in the 1960s was on January 21, 1962, almost two years earlier. but the snow fell in the Sunset and Westlake Districts, There was no snow in the financial district in any year of the sixties.
When the boy is shooting the rock in his slingshot the car is broadside to him, even a little past when he releases the shot. The rock still manages to strike the windscreen, which is impossible from that angle.
The goof item below may give away important plot points.
The car outside Gail's apartment in Rome had a red star and circle painted on it's roof. When Mr. Chase is tapped by the car that tells him to follow, he pulls out and the red paint is no longer seen, the roof is again white.