There were actually two Porsche 356 Speedsters used in the filming of this movie. The second car was a 1954 Speedster 356 serial # 80076. The front license plate switches from above to below the bumper in many scenes.
The front license plate on Harper's Porsche changes locations from above the bumper to below in the middle of the opening scene.
When Harper and Albert Graves are in the black 1965 Lincoln, the interior shots are of a 1963 Lincoln. This is evident by the different contour of the rear seat roof area, and the 1963 Lincoln dashboard (which is 2 pods), a 1965 Lincoln dashboard is one continuous panel.
When Allen Taggert tosses out the ransom money from Albert Graves' car, the rear projection screen is a city street with other cars on the street. When Harper watches Eddie Rossiter and Betty Fraley drive in and/or out of the money pickup area, it is a dirt road between some oil derricks.
When having drinks at a restaurant, Fay Estabrook kisses Lew Harper on the cheek and leaves a bright red lipstick kiss print, which he tries to ignore. A bit later, Harper spots Allan Taggert in the lobby and goes to talk to him. Taggert immediately points out the lipstick mark, which is now in a different part of Newman's cheek than where it was before.
In the opening sequence, Harper's TV is on and showing a test pattern for Channel 7. At the time, most TV stations weren't on the air all night and showed a test pattern for a few minutes immediately before sign on in the morning. When Harper opened the blind, there was bright sunshine, indicating that the sun had been up for some time. But a TV station in a large market like L.A. would already have been on the air and, thus, not showing a test pattern.
Although Roy Jenson is erroneously credited as "Roy Jensen" in the opening credits, in the end credits his name is spelled correctly.
When Harper is first talking to Mrs. Sampson, a paralytic, she moves her legs under the towel that is over her.
After Harper (played by a stuntman) dives through the shed window, he runs between some dilapidated ship vent stacks. As the stuntman moves behind the stack, one can see Harper's (Paul Newman) head sticking out on the other side and the stuntman's hind side out the other. The size of the combined Harper at that point is probably more than10 feet tall.
There are poorly meshed backgrounds whenever a front view of the characters are seen driving their cars.
When Harper and Fay are dancing to the generic music at the first bar, the lead instrument in that music is a trumpet. The band in the background consists of only guitars.
At the temple, when Claude and the immigrant laborers get the drop on Harper, Harper has fired all five shots from his .38 caliber revolver. At one point, Dwight Troy, one of the main villains, threatens Harper with that same revolver, which Harper would realize is empty, but he behaves as though it isn't.